Postfix Frequently Asked Questions

Note: this web page is no longer maintained. It exists only to avoid breaking links in web pages that describe earlier versions of the Postfix mail system.

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Table of contents

Postfix warnings and error messages

Example configurations

Sendmail incompatibility

Running hundreds of Postfix processes

Postfix performance

Receiving mail via the network

Mail relaying

Remote delivery

Local (non-virtual) delivery

Mailing lists

Virtual domains

Address rewriting

Content filtering

Other transports: UUCP, FAX, etc.

Postfix queue maintenance

Compiling and installing Postfix

Problems with specific Operating Systems

Problems with Compaq

Problems with IRIX

POP or IMAP problems

Postfix is a mail delivery system. Postfix does not implement services such as POP or IMAP to read mail. Several POP/IMAP implementations exist that can cooperate with software such as Postfix.

Examples of software that is used successfully with Postfix:

Stand-alone machine

Out of the box, Postfix should work without change on a stand-alone machine that has direct Internet access. At least, that is how Postfix installs when you download the Postfix source code. If you are on a firewalled intranet, or if your machine is dial-up connected only a small part of the time, see the respective sections.

Workstations and servers

This section describes a workstation-server environment. All systems send mail as user@domain. All systems receive mail for user@hostname. The server receives mail for user@domain, too.

Postfix has sane defaults for all parameters, so the text below shows only the overrides. In particular, Postfix will relay mail only from clients in its own subnetworks. The file (somewhat like inetd.conf) needs tweaking only if you have a very slow or a very fast network and/or machine.


        myorigin = $mydomain


        myorigin = $mydomain
        mydestination = $myhostname, localhost.$mydomain, $mydomain

In an environment like this. either the mail spool directory is shared via NFS, users access their mailboxes via POP, or each user receives her mail on her own workstation. In the latter case, each user has an alias on the server that forwards mail to the respective workstation:


        joe:    joe@joes.workstation
        jane:   jane@janes.workstation

On some systems the alias database is not in /etc/aliases. To find out the location for your system, execute the command postconf alias_maps.

Null clients

A null client is a machine that can only send mail. It receives no mail from the network, and it does not deliver any mail locally. A null client typically uses POP or NFS for mailbox access.

In the following example, mail is sent as user@domain, and all mail is forwarded to the mail server that is responsible for the local domain.

        myorigin = $mydomain
        relayhost = $mydomain
	local_transport = error:local delivery is disabled

        Comment out the SMTP server entry
        Comment out the local delivery agent entry

Since everything sends mail as user@domain, nothing sends mail as user@nullclient, and therefore no special configuration needs to be done on the mail server for mail addressed to user@nullclient.

Running Postfix inside an intranet

The simplest way to set up Postfix on a host inside a firewalled network is to send all your mail to an intranet mail gateway, and to let that mail gateway take care of forwarding.

Running Postfix on a firewall

Note: this text applies to Postfix versions 2.0 and later. To find out what Postfix version you have, execute the command postconf mail_version.

How to set up Postfix on the firewall machine so that it relays mail for to a gateway machine on the inside, and so that it refuses mail for * The problem is that the default relay_domains mail relaying restriction allows mail to * when you specify

Running Postfix on a dialup machine

This section applies to dialup connections that are down most of the time. For dialup connections that are up 24x7, see the workstations and servers section instead.

If you do not have your own hostname (as with dynamic IP addressing) and must send mail as, you should also study the the section on delivering some users locally while sending mail as user@domain.

Postfix breaks "sendmail -v"

Some people will complain that sendmail -v no longer shows the actual mail delivery.

With a distributed mail system such as Postfix, this is difficult to implement. Unlike sendmail, no Postfix mail delivery process runs under control by a user. Instead, Postfix delivers mail with daemon processes that have no parent-child relationship with user processes. This eliminates a large variety of potential security exploits with environment variables, signal handlers, and with other process attributes that UNIX passes on from parent process to child process.

Postfix uses multiple processes in order to insulate subsystems from each other. Making the delivery agents talk directly to user processes would defeat a lot of the effort that went into making Postfix more secure than ordinary mailers.

Postfix sends no "delayed mail" notices

When I was using Sendmail, after 4 hours, it would always send a receipt back to the sender saying mail delivery is delayed.

In order to make Postfix send "delayed mail" notifications after four hours, specify:

        delay_warning_time = 4h

With Postfix, delayed mail notices are turned off by default - people get enough mail already.

Postfix sends duplicate mail

Some people will complain that Postfix sends duplicate messages. This happens whenever one message is mailed to multiple addresses that reach the same user. Examples of such scenarios are:

Some people will even argue that this is the "right" behavior. It is probably more a matter of expectation and of what one is used to.

This can be "fixed" only by making Postfix slower. In the above examples, Postfix would first have to completely expand all distribution lists before starting any delivery. By design, Postfix delivers mail to different destinations in parallel, and local delivery is no exception. This is why Postfix can be faster than sendmail.

Postfix sends mail to every member of a distribution list

Some people will complain that Postfix sends mail to every member of a distribution list, including the poster. By default, Sendmail deletes the poster from distribution lists. Sendmail sends mail to the poster only when the "metoo" flag is explicitly turned on.

Wietse believes that Postfix implements the "right" behavior, and suspects that Sendmail's default behavior is a remnant from a dark past when Sendmail used some obscure algorithm to avoid aliasing loops.

Postfix ignores the owner-list alias

Normally, when a local alias foo has a companion alias owner-foo, Postfix reports delivery errors to the owner address rather than the message originator.

However, as a result of a Postfix implementation artefact, the owner-foo alias takes effect only after the alias expansion is completed.

Delivery problems that happen while expanding the alias, including delivery to commands or files, are reported to the original sender envelope address.

The reason is that bounces are sent by the Postfix queue manager, which does not know that the sender address is being replaced.

This limitation will be fixed by changing how the Postfix local delivery agent deals with undeliverable mail.

What does "fatal: open database /etc/aliases.db" mean?

DB files are maintained by the Berkeley DB library. The above message means one of the following things:

sendmail has set-uid root file permissions, or is run from a set-uid root process

Traditionally, the UNIX sendmail command is installed with set-uid root permissions. Even many MTAs other than Sendmail ship with a set-uid root sendmail command. This is not the case with Postfix. The Postfix sendmail command is designed not to be set-uid.

Unfortunately, some Linux systems have a helpful utility called linuxconf that automatically "fixes" file permissions to what they are supposed to be for Sendmail's sendmail command. Even when you reset the set-uid bit on the Postfix sendmail executable file, linuxconf will happily turn it on again for you.

On SuSE systems the file permission fixing utulity is called SuSEconfig. Other Linux systems may use different names. The usual disclaimers about mileages etc. apply.


sendmail: unable to find out your login name

This message is logged when submitting mail from a process with a userid that does not exist in the UNIX password file. Postfix uses this information in order to set the envelope sender address.

The envelope sender address is also the default value for the From: header address, when none is specified in the message.

To fix, specify the envelope sender address on the sendmail command line:

sendmail -f user@domain ...

Running hundreds of Postfix processes on FreeBSD

With hundreds of Postfix processes, the kernel will eventually run out of file handles; after that, it will run out of sockets.

To set the following kernel parameters at boot time, add the following lines to the /boot/loader.conf file (this is verified with FreeBSD 4.4):


With FreeBSD 4.2, the last three parameters cannot be set from /boot/loader.conf. To set the open file limits, execute the following commands as root:

# sysctl -w kern.maxfiles=16384
# sysctl -w kern.maxfilesperproc=16384

With FreeBSD 4.2, kern.maxproc can be set only by recompiling the kernel with a different maxusers setting in the kernel configuration file.

Running hundreds of Postfix processes on Linux

When you increase the number of Postfix processes into the hundreds, the kernel will eventually run out of file handles; after that it is likely to run out of process slots.

The following information is kernel version dependent.

To set parameters at boot time on Linux systems that have /etc/sysctl.conf, add the following lines:

fs.file-max = 16384
kernel.threads-max = 2048

To set kernel parameters at run time, execute the following commands as root:

# echo 16384 > /proc/sys/fs/file-max
# echo 2048 > /proc/sys/kernel/threads-max

Running hundreds of Postfix processes on Solaris

In order to run hundreds of processes you may have to adjust the per-process open file limit. According to the Solaris FAQ, add the following lines to /etc/system on Solaris 2.4 and later:

* set hard limit on file descriptors
set rlim_fd_max = 4096
* set soft limit on file descriptors
set rlim_fd_cur = 2048

Running thousands of Postfix delivery agents

In order to run Postfix with more than a thousand delivery agents you need to recompile the software with an appropriate value of the FD_SETSIZE constant.

% make tidy
% make makefiles "CCARGS=-DFD_SETSIZE=2048"
% make

Mail stays queued in the incoming queue

I have lots if mail in the incoming queue, but Postfix only runs a few outbound SMTP deliveries. Why is it not running more SMTP clients?

Your problem could be one of several.

Postfix responds slowly to incoming SMTP connections

My Postfix server is too slow. When I telnet to the SMTP port (telnet hostname 25), the response comes after 40 seconds. On the other hand, when I telnet to the POP port (telnet hostname 110) the response comes with no delay.


1) You need to configure Postfix to run more SMTP server processes. Edit the smtpd entry in the file and asjust the process limit, or increase the default_process_limit setting in the file. Issue the command postfix reload to make the change effective.

2) You have a name service problem.

Postfix calls the C library routines gethostbyname() and gethostbyaddr() in order to find out the SMTP client hostname. These library routines use several system configuration files in order to satisfy the request. They may in fact end up calling the DNS for reasons that are not under control by Postfix.

Depending on your system, these controlling files can be named /etc/nsswitch.conf, /etc/svcorder, /etc/host.conf or otherwise. Those files specify whether the C library routines will use local /etc/hosts before or after DNS.

Postfix logs SMTP clients as IP addresses

The Postfix SMTP server logs client connections with numerical IP addresses instead of resolving the hostname. When I use nslookup the address does resolve to a name.

You run the Postfix SMTP server inside a chroot jail for extra security, but some configuration files are missing or have incorrect information. The command "postfix check" will report what files may have incorrect information. For example:

warning: /var/spool/postfix/etc/resolv.conf and /etc/resolv.conf differ
warning: /var/spool/postfix/etc/localtime and /etc/localtime differ

In order to run inside a chroot jail, the Postfix SMTP client and server need copies of system configuration files inside the Postfix queue directory. The exact list of files is very system dependent, but you will probably need at the very least:


Of course, these directories and files must be owned by root, but they must be accessible by the postfix user, so directories need mode 0755 and files need mode 0644.

For more details, see the files in the examples/chroot-setup directory of the Postfix source code distribution.

warning: address not listed for hostname yyy.yyy.yyy

Postfix uses hostnames in its junk mail and mail relay controls. This means that in theory someone could be motivated to set up bogus DNS information, in order to get past your junk mail or mail relay controls.

When Postfix looks up the SMTP client hostname for the SMTP client IP address, then Postfix also checks if the SMTP client IP address is listed under the SMTP client hostname.

If the SMTP client IP address is not listed under the SMTP client hostname, then Postfix concludes that the SMTP client hostname does not belong to the SMTP client IP address, and ignores the SMTP client hostname. A warning is logged, so that you can find out why an SMTP client is or is not stopped by your junk mail or mail relay checks.

You could contact the people who maintain the SMTP client's DNS records, and explain to them that each IP address needs one PTR record, and that this one PTR record needs a matching A record.

Some people read the RFCs such that one IP address can have multiple PTR records, but that makes PTR records even less useful than they already are. And in any case, having multiple names per IP address only worsens the problem of finding out the SMTP client hostname.

Relaying mail for mobile users

I have Postfix setup on a machine but I'd like to have a select group of Internet users be able to relay mail through it. I'd either like to base the relaying on IP address (e.g., a 256-block for dynamic IP people) or on hostname (

The most preferable way is to have users submit mail via some authenticated protocol instead of plain old SMTP.

The next best way is to use plain old SMTP and to authenticate the user first, for example, with a "please login via POP before using SMTP" scheme. In that case, some software maintains a Postfix-compatible access table with client IP address information.

        smtpd_recipient_restrictions =
            check_client_access hash:/etc/postfix/client_access

    /etc/postfix/client_access:         OK         987654321

Specify dbm instead of hash if your system uses dbm files instead of db files. To find out what map types Postfix supports, use the command postconf -m.

N.B. Some non-Postfix software uses btree files instead of hash files. In that case, you will have to adjust the above check_client_access restriction accordingly.

A less preferable way is based on client IP address (for example, a 256-block) or DNS hostname (for example, This scheme does not authenticate the user. If you use IP/DNS-based relay access control, pray that no customer with that same ISP points their spam software at your machine, or else you may end up on internet-wide black lists.

The least preferable way is based on the sender address. It is trivially easy to spoof by anyone who ever received mail from your site. If you use sender address access control, pray that no spammer ever finds out the address of your users.

        smtpd_recipient_restrictions =
            check_client_access hash:/etc/postfix/client_access
            check_sender_access hash:/etc/postfix/sender_access

        11.22.33                OK          OK

        joe@my.domain           OK
        blow@my.domain          OK

Restricting what users can send mail to off-site destinations

How can I configure Postfix in a way that some users can send mail to the internet and other users not. The users with no access should receive a generic bounce message. Please don't discuss whether such access restrictions are necessary, it was not my decision.

Postfix has support for per-user restrictions. The restrictions are implemented by the SMTP server. Thus, users that violate the policy have their mail rejected by the SMTP server. Like this:

554 <user@remote>: Access denied

The implementation uses two lookup tables. One table defines what users are restricted in where they can send mail, and the other table defines what destinations are local. It is left as an exercise for the reader to change this into a scheme where only some users have permission to send mail to off-site destinations, and where most users are restricted.

The example assumes DB/DBM files, but this could also be done with LDAP or SQL.

        smtpd_recipient_restrictions =
            check_sender_access hash:/etc/postfix/restricted_senders
            ...other stuff...

        smtpd_restriction_classes = local_only
        local_only = check_recipient_access hash:/etc/postfix/local_domains, reject

        foo@domain      local_only
        bar@domain      local_only

        this.domain     OK      matches this.domain and subdomains
        that.domain     OK      matches that.domain and subdomains

Specify dbm instead of hash if your system uses dbm files instead of db files. To find out what map types Postfix supports, use the command postconf -m.

The smtpd_restriction_classes verbiage exists so that Postfix can open /etc/postfix/local_domains.db before entering a chroot jail, so it is only an artefact of implementation.

This scheme does not authenticate the user, therefore it can be bypassed in several ways:

Configuring Postfix as MX host for a remote site

When you are secondary mx for a remote site this is all you need:

        the.backed-up.domain.tld        IN      MX 100 your.machine.tld

        relay_domains = $mydestination the.backed-up.domain.tld
        smtpd_recipient_restrictions = permit_mynetworks reject_unauth_destination

DO NOT LIST the.backed-up.domain.tld in MYDESTINATION

DO NOT LIST the.backed-up.domain.tld as a VIRTUAL DOMAIN

When you are primary mx for a remote site you also need:

        transport_maps = hash:/etc/postfix/transport

        the.backed-up.domain.tld       relay:[]

Specify dbm instead of hash if your system uses dbm files instead of db files. To find out what map types Postfix supports, use the command postconf -m.

Mail stays queued with: Host not found, try again

When I send mail to a remote address, the following happens:

    Jul 14 12:45:38 myhostname postfix/qmgr[2246]: 74FBF30501:
        from=<sender@sender.domain> size=309 (queue active)
    Jul 14 12:45:39 myhostname postfix/smtp[2349]: 74FBF30501:
        to=<recip@recip.domain> relay=none, delay=3944,
        status=deferred (Name service error for name=recip.domain
        type=MX: Host not found, try again)

However, I can nslookup the hostname just fine.

There can be several different problems.

Mail fails consistently with timeout or lost connection

Every now and then, mail fails with "timed out while sending end of data -- message may be sent more than once", or with: "lost connection after DATA". Network outages happen, systems crash. There isn't much you can do about it. Usually the problem goes away by itself.

However, when you see mail deliveries fail consistently, you may have a different problem: broken path MTU discovery. Or it could be a broken PIX firewall.

Cisco PIX "fixup protocol smtp" bug

The Cisco PIX firewall has a bug when running software older than version 5.2(4) or 6.0(1).

The bug ID is CSCds90792. The "fixup protocol smtp" feature does not correctly handle the case where the "." and the "CRLF" at the end of mail are sent in separate packets.

How does one recognize a mailer behind a Cisco PIX with "fixup protocol smtp" enabled? As of version 5.1 and later, the fixup protocol smtp command changes the characters in the SMTP banner to asterisks except for the "2", "0" and "0 SPACE" characters.

When you connect to a mailer behind such a filter you see something like:

220 **************************************0******0*********20 ****200**0*********0*00

IP path MTU discovery

A little background is in order. With the SMTP protocol, the HELO, MAIL FROM and RCPT TO commands and responses are relatively short. When you're talking to old versions of sendmail, every command and every response is sent as a separate packet, because sendmail didn't implement ESMTP command pipelining until recently.

The message content, however, is sent as a few datagrams, each datagram typically a kbyte large or even bigger, depending on your local network MTU.

When mail fails consistently due to a timeout, I suspect that the sending machine runs a modern UNIX which implements path MTU discovery. That causes the machine to send packets as large as it would send over the LAN, with the IP DON'T FRAGMENT bit set, preventing intermediate routers from fragmenting the packets that are too big for their networks.

Depending on what network path a message follows, some router on the way responds with an ICMP MUST FRAGMENT message saying the packet is too big. Normally, the sending machine will re-send the data after chopping it up into smaller pieces.

However, things break when some router closer to the sending system is dropping such ICMP feedback messages, in a mistaken attempt to protect systems against certain attacks. In that case, the ICMP feedback message never reaches the sending machine, and the connection times out.

This is the same configuration problem that causes trouble with web servers behind a misconfigured packet filter: small images/files are sent intact, large images/files time out because the server does not see the MUST FRAGMENT ICMP feedback messages.

Workaround: at the sending machine, disable path MTU discovery. Mail will get out, but of course everyone else will still suffer. How to disable path MTU discovery? It depends. Solaris has an ndd command; other systems use different means such as sysctl to control kernel parameters on a running system.

Workaround: at the receiving machine, set a smaller MTU. For example, people using PPPoE (PPP over Ethernet) often have to choose an MTU lightly smaller than the default 1500 for ethernet.

Fix: find the router that drops the ICMP MUST FRAGMENT messages, and convince the person responsible for it to fix the configuration.

Postfix does not try all the MX addresses

When delivering mail, Postfix tries all MX addresses in order of preference, and stops at the first server that speaks SMTP. However, once an SMTP greeting is received, Postfix will not move on to the next MX host if the delivery fails.

This will eventually be solved when Postfix implements SMTP connection caching.

What does "fatal: unknown service: smtp/tcp" mean?

Your Postfix /etc/postfix/ file specifies that the Postfix SMTP client runs inside a chroot environment. However, the files necessary for that mode of operation are not installed below /var/spool/postfix.

Enabling chroot operation adds a non-trivial barrier for system penetrators.

Two solutions:

Mail delivery fails with: "unknown mail transport error"

This is an opportunity to meet your friends egrep and less. Postfix activity, including progres and failure, is logged to a logfile, typically named /var/log/maillog. To find out where Postfix activity is logged on your machine, examine the /etc/syslog.conf file.

To find out the cause for the "unknown mail transport error", type the following command:

egrep '(warning|fatal|panic):' /var/log/maillog | less
Pay particular attention to messages that are labeled as fatal and panic. These describe catastrophic failures that need to be addressed before Postfix is happy. Problems labeled as fatal are fixed by you, by adjusting configuration files, file permissions and so on. Problems labeled as panic are fixed by the Postfix author, by changing Postfix source code.

Root's mail is delivered to nobody

If you use
procmail (or some other command) for local mail delivery, Postfix will not deliver mail as root. Instead, Postfix runs procmail (or whatever) as nobody. Perhaps some day Wietse will trust Postfix enough to run external commands as root.

Solution: just like you're not supposed to log in as root (except for unusual conditions), you're not supposed to receive mail as root.

On some systems the alias database is not in /etc/aliases. To find out the location for your system, execute the command postconf alias_maps.

What does "biff_notify: Connection refused" mean?

By default, the Postfix local delivery agent attempts to notify local users of the arrival of new mail. This feature makes use of the comsat network service, which is turned off on many UNIX systems for performance and/or security reasons.

The Postfix warning message means that new mail notification failed because the comsat network service is turned off.

To disable the comsat client code in the Postfix delivery agent, specify:

    biff = no

Note: recent versions of procmail also produce biff notifications. To silence biff completely you may also have to update procmail configuration files.

To enable the comsat network service, uncomment the corresponding entry in the inetd.conf file, and kill -HUP the inetd process.

What does "NIS domain name not set - NIS lookups disabled" mean?

The warning message means that NIS (Network Information Service) is not enabled on your machine. That is perfectly OK. It's just hard for Postfix to find out about these things ahead of time.

To disable the NIS client code in the Postfix local delivery agent, update the corresponding section in the file and specify one of the following, depending on the type of aliases file:

    alias_maps = $alias_database

This forces Postfix to use only the local aliases database, if one is defined.

Postfix rejects mail with "User unknown in local recipient table"

As of version Postfix 2.0, you are expected to tell the Postfix SMTP server what local users exist by listing all tables with local usernames or addresses in the local_recipient_maps parameter. To find out what Postfix version you have, execute the command postconf mail_version.

The default local_recipient_maps setting assumes that you use the default Postfix local delivery agent:

        local_recipient_maps = $alias_maps, proxy:unix:passwd.byname

You need the proxy: part only if specifies that the Postfix SMTP server runs chrooted. As distributed by the author, Postfix runs no daemons chrooted.

The local recipients tables are searched by the recipient address (user@domain) and by the recipient name (the address minus the domain). Postfix does not care what the lookup result looks like, so you can use any database that Postfix understands the format of.

To stop Postfix from rejecting local mail incorrectly:

To disable the local_recipient_maps feature, specify:

        local_recipient_maps = 

With this setting, the Postfix SMTP server will not reject mail for unknown local recipients.

Delivering some users locally while sending mail as user@domain

Support for maildir-style mailboxes

Maildir is a specific one-file-per-message organization that was introduced with the qmail system by Daniel Bernstein. In order to turn on maildir-style delivery, specify, for example:

        home_mailbox = Maildir/

Any relative pathname that ends in / turns on maildir delivery. The home_mailbox value is appended to the user's home directory pathname.

The maildir format is also supported with delivery via aliases or via .forward files. Specify /file/name/ as destination. The trailing / turns on maildir delivery.

Using Procmail for system-wide local delivery

Warning: if you use procmail in this manner, you must set up an alias for root that forwards mail for root to a real user. See the FAQ entry titled "Mail for root is delivered to nobody". Postfix exports information via environment variables. The contents are censored. Characters that may have special meaning to the shell, including whitespace, are replaced by underscores.

The text to the right-hand side of the @ in the recipient address.
Optional address extension part.
The recipient's home directory.
The text to the left-hand side of the @ in the recipient address, for example, $USER+$EXTENSION.
The recipient username.
The entire recipient address, $LOCAL@$DOMAIN.
The complete sender address.
The recipient's login shell.
The recipient username.

What does "warning: cannot access UNIX password database" mean?

This message is logged when, for example, the Postfix SMTP server is unable to access the UNIX password database.

Getting rid of the ugly Delivered-To: header

Some people will complain about the ugly Delivered-To: message header that Postfix prepends to their mail. By default, Postfix prepends this header when forwarding mail, and when delivering to file (mailbox) or command. The purpose is to stop mail forwarding loops as early as possible, that is, before they have a chance to happen. But the header is ugly, no question about it.

Solutions, ranging from fighting symptoms to turning off the Delivered-To: header:

See also the FAQ item for problems with the majordomo approve command.

Postfix breaks the majordomo "approve" command

The Postfix local delivery agent prepends a Delivered-To: message header to prevent mail forwarding loops. With majordomo mailing lists, Delivered-To: gets in the way when the moderator wants to approve postings that were sent to the list. The Postfix system claims that the mail is looping.

Currently, the recommended workaround is to edit the approve script to strip any header lines that match:


Yes, this assumes that the moderator knows what she is doing.

A less-preferred workaround is to not insert Delivered-To: when delivering to commands such as majordomo. See the FAQ entry titled "Getting rid of the ugly Delivered-To: header".

Postfix accepts MAIL FROM and RCPT TO "| command"

With Postfix, | or / has special meaning only when it appears in aliases, .forward files or in :include: files. It has no special meaning in mail addresses.

If you must receive mail for systems with 10-year old vulnerabilities, it is prudent to set up a regexp filter that rejects potentially harmful MAIL FROM or RCPT TO commands.

        smtpd_sender_restrictions =
        smtpd_recipient_restrictions =

        /[/|]/  REJECT

However, rejecting all envelope addresses with / causes trouble with simple-minded X.400 to Internet address mappings that leave the X.400 address structure exposed.

See also the documentation on header checks restrictions for message header contents. These restrictions can be used to protect against attacks with command/file destinations in, for example, Errors-To: or Return-Receipt_To: message headers.

Protecting internal email distribution lists

We want to implement an internal email distribution list. Something like, which aliases to all employees. My first thought was to use the aliases map, but that would lead to "all" being accessible from the "outside", and this is not desired... :-)
Postfix can implement per-address access controls. What follows is based on the SMTP client IP address, and therefore is subject to IP spoofing.

        smtpd_recipient_restrictions =
            ..the usual stuff...

        all     permit_mynetworks,reject

Specify dbm instead of hash if your system uses dbm files instead of db files. To find out what map types Postfix supports, use the command postconf -m.

Now, that would be sufficient when your machine receives all Internet mail directly from the Internet. That's unlikely if your network is a bit larger than an office. For example your backup MX hosts would "launder" the client IP address of mail from outside so it would appear to come from a trusted machine.

In the general case you need two lookup tables: one table that lists destinations that need to be protected, and one table that lists domains that are allowed to send to the protected destinations.

What follows is based on the sender SMTP envelope address, and therefore is subject to SMTP sender spoofing.

        smtpd_recipient_restrictions =
            ..the usual stuff...

        smtpd_restriction_classes = insiders_only
        insiders_only = check_sender_access hash:/etc/postfix/insiders, reject

        all@my.domain   insiders_only
        all@my.hostname insiders_only

        my.domain       OK
        another.domain  OK

The smtpd_restriction_classes verbiage is needed so that Postfix knows what lookup tables to open before it goes to chroot jail. It is only an artefact of the implementation.

Getting past this scheme is relatively easy, because all one has to do is to spoof the SMTP sender address.

If the internal list is a low-volume one, perhaps it makes more sense to make it moderated.

Postfix rejects mail with "User unknown in virtual alias table"

Answer: you have listed the virtual domain name in the tables specified with the virtual_alias_domains parameter, but the recipient address is not listed in the tables specified with the virtual_alias_maps parameter.

If you want to deliver the domain via the Postfix virtual(8) mailbox delivery agent, then you should list the virtual domain name in the tables specified with the virtual_mailbox_domains parameter instead.

Postfix rejects mail with "User unknown in virtual mailbox table"

Answer: you have listed the virtual domain name in the tables specified with the virtual_mailbox_domains parameter, but the recipient address is not listed in the tables specified with the virtual_mailbox_maps parameter.

If you want to deliver the domain as a virtual(5) alias domain, where each address is aliased to a real local or remote address, then you should list the virtual domain name in the tables specified with the virtual_alias_domains parameter instead.

Postfix does not refuse mail for unknown users in virtual domains

Mail for unknown users in a virtual domain fails with "mail loops back to myself"

Postfix refuses mail for virtual domains with "relay access denied"


Commands, mailing lists, and /file/name destinations don't work in virtual domains

Quick answer: set up "punch through" virtual aliases that redirect the mail to local Postfix aliases:

        virtual_alias_maps = hash:/etc/postfix/virtual

        listname@virtual.tld            listname
        owner-listname@virtual.tld      owner-listname
        listname-request@virtual.tld    listname-request

        listname: "|whatever"
        owner-listname: user@domain
        listname-request: "|whatever"

This redirects mail for virtual address listname@virtual.tld etc. to local address listname@your.domain.tld etc.. Mail for these local aliases is then delivered to external commands or files etc. by the Postfix local delivery agent.

Long answer:

Delivering mail to a file or command is a security-sensitive operation, because the operation must be executed with the right privileges. Only root-privileged software such as the Postfix local delivery agent can set the privileges for command or file delivery.

For security reasons, Postfix tries to avoid using root privileges where possible. In particular, Postfix virtual mapping is done by an unprivileged daemon, so there is no secure way to execute commands or to deliver to files specified in virtual maps.

Receiving a virtual domain in a mailbox

Question: how to receive all mail for a domain in a mailbox without losing the original recipient information? The Postfix Delivered-To: mail header shows only the mailbox owner, not the virtual address that the mail was sent to.

Answer: Postfix logs the original recipient address in the X-Original-To: message header.

Address masquerading with exceptions

For people outside your organization it can be desirable to only see addresses of the form rather than addresses with individual internal host names. This can be achieved with address masquerading.

Address masquerading is intended for use only on mail gateways.

In some cases, you may wish to have certain users or hosts exempted from masquerading.

As usual, execute the command postfix reload to make the changes effective.

What does "Error: too many hops" mean?

Short answer: this message means that mail is probably looping. If you see this after you turned on Postfix content filtering, then you have made a mistake that causes mail to be filtered repeatedly. This is cured by appropriate use of content_filter=, header_checks=, and body_checks=.

Long answer: the message has too many Received: message headers. A received header is added whenever Postfix (or any MTA) receives a message. A large number of Received: message headers is an indication that mail is looping around.

Side comment: email uses the opposite of the technique that is used to avoid IP forwarding loops. With IP, the sender sets a TTL (time to live) field in the IP header. The field is decremented by each router. When the TTL reaches zero the packet is discarded and an ICMP error message is returned to the sender.

Using UUCP over TCP

This subject comes up whenever someone asks about a "domain in a mailbox" solution. For first-hand information, see the guides listed below.

Setting up an Internet to UUCP gateway

Here is how to set up a machine that sits on the Internet and that delivers some but not all non-local mail via UUCP. See the UUCP-only FAQ entry for setting a UUCP-only host.

Using UUCP as the default transport

Here is how to relay all your mail over a UUCP link. See the Internet to UUCP FAQ entry for setting up a machine that gateways between UUCP and SMTP.

Sending mail to a FAX machine

The following information is by Joerg Henne:

Over here we are using the scheme <fax number>@fax.our.domain with Postfix and HylaFax. Here's the setup used:

        fax       unix  -       n       n       -       1       pipe
            flags= user=fax argv=/usr/bin/faxmail -d -n ${user}

        fax.your.domain   fax:localhost

        transport_maps = hash:/etc/postfix/transport
        fax_destination_recipient_limit = 1

The process limit of 1 in the file is necessary with fax software that cannot handle multiple requests at the same time. It won't hurt otherwise.

The fax_destination_recipient_limit entry (by Simon, Mr. Simix) is necessary with fax software that can't have more than one destination on its command line. It won't hurt otherwise.

Specify dbm instead of hash if your system uses dbm files instead of db files. To find out what map types Postfix supports, use the command postconf -m.

Note: be sure to not advertise fax.your.domain in the DNS :-)

Deleting a message from the Postfix queue

The postsuper command has an option to delete Postfix message queue files. To delete the message with queue id ABCDEF, perhaps obtained from mailq output, one would use:

# postsuper -d ABCDEF

To delete a large number of files one would use:

# postsuper -d - < filename-with-queue-ids

It is usually safe to do this while the Postfix system is running. However, there is a small chance of deleting the wrong queue file. The scenario goes like this:

Moving or restoring the Postfix queue

It is not safe to simply copy Postfix queue files from one file system (or backup) to another file system. The reason for this is that queue file names must be unique across the Postfix incoming, active and deferred queue directories. If two queue files have the same file (base) name, then one of the queue files may be lost as files are moved from queue directory to queue directory.

Postfix names a queue file after its inode number and after the microsecond part of the time of day. Thus, if a queue file has a name based on someone elses inode number there is a small chance that the file name will collide with another queue file.

The text below describes two different procedures to restore queue files from another machine or from backup.

Procedure 1: Your Postfix queue is empty, and you run Postfix release 20010525 or later

Procedure 2: Your Postfix queue is not empty, or you are running a Postfix release prior to 20010525

Undefined symbols: ___dn_expand, ___res_init etc.

Question: When I build Postfix I get the following errors:

    ld: Undefined symbol
    *** Error code 1

Answer: you're mixing BIND version 8 include files with a different version of the resolver library.

Fix: use the right include files. For example:

    make makefiles CCARGS="-I/usr/include".

Undefined symbols: dbm_pagfno, dbm_dirfno etc.

Question: When I build Postfix I get the following errors:

    Undefined                       first referenced
     symbol                             in file
       dbm_pagfno                          ../lib/libutil.a(dict_dbm.o)
       dbm_dirfno                          ../lib/libutil.a(dict_dbm.o)

Answer: instead of using /usr/include/ndbm.h, you're building Postfix with some incompatible third-party file, typically /usr/local/include/ndbm.h.

Fix: get rid of the third-party ndbm.h include file.

Using third-party DB libraries

The old dbm UNIX database has severe limitations when you try to store lots of information. It breaks when the number of hash collisions becomes so large that the entries no longer fit together in a single disk block. The more modern db database does not suffer these limitations. It is standard on 4.4BSD and Linux systems.

In order to build Postfix with db support on UNIX systems that do not have db support out of the box, you can use the Berkeley DB source code from See the file DB_README in the Postfix source code distribution for instructions on how to build Postfix with Sleepycat's Berkeley DB.

IRIX problems translating IP address to string

While installing IRIX 6.5.7m on a clean disk and no special options or software I stumbled upon the following problem; the inet_ntoa() function seems to return INADDR_NONE (malformed request?) for every call to it.

There is an incompatibility between gcc and system libraries compiled with SGI's cc. See a description in

If you must use gcc, a possible workaround is to use the inet_ntoa() routine from the BIND source code at

Compaq mail blackhole problem

On some Compaq Tru64 UNIX configurations, Postfix will receive mail and then nothing happens. The mail does not even show up with the mailq command.

Postfix sets the execute bit on a queue file to indicate that it is done receiving a message. As long as a queue file does not have the execute bit set, Postfix will ignore it as "mail still being received".

With enhanced security enabled, Compaq Tru64 UNIX has a feature that disallows non-superuser attempts to set the execute bit on a queuefile. Unfortunately, Postfix is never informed that such attempts fail, and mail seems to disappear into a black hole.

Postfix could be modified to use some other bit than the execute bit, but that might equally well fail on other systems. Another possibility is to allow non-superusers to set the execute bit on files, and to mount the Postfix queue file system with the noexec option or equivalent.

Too many connections

This message is produced by the MYSQL server. You need to increase the number of connections that it can handle. Things to bear in mind: the virtual and canonical maps are accessed by every smtpd and cleanup process.

write queue file: No such file or directory

write queue file: Unknown error 4294967289

Reiserfs reports the wrong error code when a message exceeds the message_size_limit setting. As a result, the Postfix SMTP server reports a "queue file write error" to the SMTP client, rather than reporting a "file too large" condition. The client will keep sending the same email again and again until the mail is too old.
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