Postfix Address Verification Howto


WARNING

Recipient address verification may cause an increased load on down-stream servers in the case of a dictionary attack or a flood of backscatter bounces. Sender address verification may cause your site to be blacklisted by some providers. See also the "Limitations" section below for more.

What Postfix address verification can do for you

Address verification is a feature that allows the Postfix SMTP server to block a sender (MAIL FROM) or recipient (RCPT TO) address until the address has been verified to be deliverable.

The technique has obvious uses to reject junk mail with an unreplyable sender address.

The technique is also useful to block mail for undeliverable recipients, for example on a mail relay host that does not have a list of all the valid recipient addresses. This prevents undeliverable junk mail from entering the queue, so that Postfix doesn't have to waste resources trying to send MAILER-DAEMON messages back.

This feature is available in Postfix version 2.1 and later.

Topics covered in this document:

How address verification works

A Postfix MTA verifies a sender or recipient address by probing the preferred MTAs for that address, without actually delivering mail. The preferred MTAs could include the Postfix MTA itself, or some remote MTAs (SMTP interruptus). Probe messages are like normal mail, except that they are never delivered, deferred or bounced; probe messages are always discarded.

  -> probe
message
-> Postfix
mail
queue
Internet -> Postfix
SMTP
server
<-> Postfix
verify
server
|
v
<- probe
status
<- Postfix
delivery
agents
-> Local
-> Remote
  ^
|
v
 
  Address
verification
database

With Postfix address verification turned on, normal mail will suffer only a short delay of up to 6 seconds while an address is being verified for the first time. Once an address status is known, the status is cached and Postfix replies immediately.

When verification takes too long the Postfix SMTP server defers the sender or recipient address with a 450 reply. Normal mail clients will connect again after some delay. The address verification delay is configurable with the main.cf address_verify_poll_count and address_verify_poll_delay parameters. See postconf(5) for details.

Limitations of address verification

Recipient address verification

As mentioned earlier, recipient address verification is useful to block mail for undeliverable recipients on a mail relay host that does not have a list of all valid recipient addresses. This can help to prevent the mail queue from filling up with MAILER-DAEMON messages.

Recipient address verification is relatively straightforward and there are no surprises. If a recipient probe fails, then Postfix rejects mail for the recipient address. If a recipient probe succeeds, then Postfix accepts mail for the recipient address. However, recipient address verification probes can increase the load on down-stream MTAs when you're being flooded by backscatter bounces, or when some spammer is mounting a dictionary attack.

By default, address verification results are saved in a persistent database (Postfix version 2.7 and later; with earlier versions, specify the database in main.cf as described later). The persistent database helps to avoid probing the same address repeatedly.

/etc/postfix/main.cf:
    smtpd_recipient_restrictions = 
        permit_mynetworks
        # reject_unauth_destination is not needed here if the mail
        # relay policy is specified under smtpd_relay_restrictions
        # (available with Postfix 2.10 and later).
        reject_unauth_destination
        ...
        reject_unknown_recipient_domain
        reject_unverified_recipient
        ...
    # Postfix 2.6 and later privacy feature.
    # unverified_recipient_reject_reason = Address lookup failed

The "reject_unknown_recipient_domain" restriction blocks mail for non-existent domains. Putting this before "reject_unverified_recipient" avoids the overhead of generating unnecessary probe messages.

The unverified_recipient_reject_code parameter (default 450) specifies the numerical Postfix SMTP server reply code when a recipient address is known to bounce. Change this setting into 550 when you trust Postfix's judgments.

The following features are available in Postfix 2.6 and later.

The unverified_recipient_defer_code parameter (default 450) specifies the numerical Postfix SMTP server reply code when a recipient address probe fails with some temporary error. Some sites insist on changing this into 250. NOTE: This change turns MX servers into backscatter sources when the load is high.

The unverified_recipient_reject_reason parameter (default: empty) specifies fixed text that Postfix will send to remote SMTP clients, instead of sending actual address verification details. Do not specify the SMTP status code or enhanced status code.

The unverified_recipient_tempfail_action parameter (default: defer_if_permit) specifies the Postfix SMTP server action when a recipient address verification probe fails with some temporary error.

Sender address verification for mail from frequently forged domains

Only for very small sites, it is relatively safe to turn on sender address verification for specific domains that often appear in forged email.

/etc/postfix/main.cf:
    smtpd_sender_restrictions = hash:/etc/postfix/sender_access
    unverified_sender_reject_code = 550
    # Postfix 2.6 and later.
    # unverified_sender_defer_code = 250

    # Default setting for Postfix 2.7 and later.
    # Note 1: Be sure to read the "Caching" section below!
    # Note 2: Avoid hash files here. Use btree instead.
    address_verify_map = btree:/var/lib/postfix/verify
 
/etc/postfix/sender_access:
    # Don't do this when you handle lots of email.
    aol.com     reject_unverified_sender
    hotmail.com reject_unverified_sender
    bigfoot.com reject_unverified_sender
    ... etcetera ...

At some point in cyberspace/time, a list of frequently forged MAIL FROM domains could be found at http://www.monkeys.com/anti-spam/filtering/sender-domain-validate.in.

NOTE: One of the first things you might want to do is to turn on sender address verification for all your own domains.

Sender address verification for all email

Unfortunately, sender address verification cannot simply be turned on for all email - you are likely to lose legitimate mail from mis-configured systems. You almost certainly will have to set up white lists for specific addresses, or even for entire domains.

To find out how sender address verification would affect your mail, specify "warn_if_reject reject_unverified_sender" so that you can see what mail would be blocked:

/etc/postfix/main.cf:
    smtpd_sender_restrictions = 
        permit_mynetworks
        ... 
        check_sender_access hash:/etc/postfix/sender_access
        reject_unknown_sender_domain
        warn_if_reject reject_unverified_sender 
        ...
    # Postfix 2.6 and later.
    # unverified_sender_reject_reason = Address verification failed

    # Default setting for Postfix 2.7 and later.
    # Note 1: Be sure to read the "Caching" section below!
    # Note 2: Avoid hash files here. Use btree instead.
    address_verify_map = btree:/var/lib/postfix/verify

This is also a good way to populate your cache with address verification results before you start to actually reject mail.

The sender_access restriction is needed to whitelist domains or addresses that are known to be OK. Although Postfix will not mark a known-to-be-good address as bad after a probe fails, it is better to be safe than sorry.

NOTE: You will have to whitelist sites such as securityfocus.com and other sites that operate mailing lists that use a different sender address for each posting (VERP). Such addresses pollute the address verification cache quickly, and generate unnecessary sender verification probes.

/etc/postfix/sender_access
    securityfocus.com OK
    ...

The "reject_unknown_sender_domain" restriction blocks mail from non-existent domains. Putting this before "reject_unverified_sender" avoids the overhead of generating unnecessary probe messages.

The unverified_sender_reject_code parameter (default 450) specifies the numerical Postfix server reply code when a sender address is known to bounce. Change this setting into 550 when you trust Postfix's judgments.

The following features are available in Postfix 2.6 and later.

The unverified_sender_defer_code parameter (default 450) specifies the numerical Postfix SMTP server reply code when a sender address verification probe fails with some temporary error. Specify a valid 2xx or 4xx code.

The unverified_sender_reject_reason parameter (default: empty) specifies fixed text that Postfix will send to remote SMTP clients, instead of sending actual address verification details. Do not specify the SMTP status code or enhanced status code.

The unverified_sender_tempfail_action parameter (default: defer_if_permit) specifies the Postfix SMTP server action when a sender address verification probe fails with some temporary error.

Address verification database

To improve performance, the Postfix verify(8) daemon can save address verification results to a persistent database. This is enabled by default with Postfix 2.7 and later. The address_verify_map (NOTE: singular) configuration parameter specifies persistent storage for sender or recipient address verification results. If you specify an empty value, all address verification results are lost after "postfix reload" or "postfix stop".

# Example 1: Default setting for Postfix 2.7 and later.
# Note: avoid hash files here. Use btree instead.
/etc/postfix/main.cf:
    address_verify_map = btree:$data_directory/verify_cache

# Example 2: Shared persistent lmdb: cache (Postfix 2.11 or later).  
# Disable automatic cache cleanup in all Postfix instances except
# for one instance that will be responsible for cache cleanup.
/etc/postfix/main.cf:
    address_verify_map = lmdb:$data_directory/verify_cache
    # address_verify_cache_cleanup_interval = 0

# Example 3: Shared persistent btree: cache (Postfix 2.9 or later).  
# Disable automatic cache cleanup in all Postfix instances except
# for one instance that will be responsible for cache cleanup.
/etc/postfix/main.cf:
    address_verify_map = proxy:btree:$data_directory/verify_cache
    # address_verify_cache_cleanup_interval = 0

# Example 4: Shared memory cache (requires Postfix 2.9 or later).
# Disable automatic cache cleanup in all Postfix instances.
# See memcache_table(5) for details.
/etc/postfix/main.cf:
    address_verify_map = memcache:/etc/postfix/verify-memcache.cf
    address_verify_cache_cleanup_interval = 0

# Example 5: Default setting for Postfix 2.6 and earlier.
# This uses non-persistent storage only.
/etc/postfix/main.cf:
    address_verify_map =

NOTE 1: The database file should be stored under a Postfix-owned directory, such as $data_directory.

As of version 2.5, Postfix no longer uses root privileges when opening this file. To maintain backwards compatibility, an attempt to open the file under a non-Postfix directory is redirected to the Postfix-owned data_directory, and a warning is logged. If you wish to continue using a pre-existing database file, change its file ownership to the account specified with the mail_owner parameter, and either move the file to the data_directory, or move it to some other Postfix-owned directory.

NOTE 2: Do not put this file in a file system that may run out of space. When the address verification table gets corrupted the world comes to an end and YOU will have to MANUALLY fix things as described in the next section. Meanwhile, you will not receive mail via SMTP.

NOTE 3: The verify(8) daemon will create a new database when none exists. It will open or create the file before entering the chroot jail.

Managing the address verification database

The verify(8) manual page describes parameters that control how long address verification results are cached before they need to be refreshed, and how long results can remain "unrefreshed" before they expire. Postfix uses different controls for positive results (address was accepted) and for negative results (address was rejected, or address verification failed for some other reason).

The verify(8) daemon will periodically remove expired entries from the address verification database, and log the number of entries retained and dropped (Postfix versions 2.7 and later). A cleanup run is logged as "partial" when the daemon terminates early because of "postfix reload, "postfix stop", or because the daemon received no requests for $max_idle seconds. Postfix versions 2.6 and earlier do not implement automatic address verification database cleanup. There, the database is managed manually as described next.

When the address verification database file becomes too big, or when it becomes corrupted, the solution is to manually rename or delete (NOT: truncate) the file and run "postfix reload". The verify(8) daemon will then create a new database file.

Controlling the routing of address verification probes

By default, Postfix sends address verification probe messages via the same route as regular mail, because that normally produces the most accurate result. It's no good to verify a local address by connecting to your own SMTP port; that just triggers all kinds of mailer loop alarms. The same is true for any destination that your machine is best MX host for: hidden domains, virtual domains, etc.

However, some sites have a complex infrastructure where mail is not sent directly to the Internet, but is instead given to an intermediate relayhost. This is a problem for address verification, because remote Internet addresses can be verified only when Postfix can access remote destinations directly.

For this reason, Postfix allows you to override the routing parameters when it delivers an address verification probe message.

First, the address_verify_relayhost parameter allows you to override the relayhost setting, and the address_verify_transport_maps parameter allows you to override the transport_maps setting. The address_verify_sender_dependent_relayhost_maps parameter does the same for sender-dependent relayhost selection.

Second, each address class is given its own address verification version of the message delivery transport, as shown in the table below. Address classes are defined in the ADDRESS_CLASS_README file.

Domain list Regular transport Verify transport
mydestination local_transport address_verify_local_transport
virtual_alias_domains (not applicable) (not applicable)
virtual_mailbox_domains virtual_transport address_verify_virtual_transport
relay_domains relay_transport address_verify_relay_transport
(not applicable) default_transport address_verify_default_transport

By default, the parameters that control delivery of address probes have the same value as the parameters that control normal mail delivery.

Forced probe routing examples

In a typical scenario one would override the relayhost setting for address verification probes and leave everything else alone:

/etc/postfix/main.cf:
    relayhost = $mydomain
    address_verify_relayhost =
    ...

Sites behind a network address translation box might have to use a different SMTP client that sends the correct hostname information:

/etc/postfix/main.cf:
    relayhost = $mydomain
    address_verify_relayhost =
    address_verify_default_transport = direct_smtp

/etc/postfix/master.cf:
    direct_smtp .. .. .. ..  .. .. .. .. .. smtp
        -o smtp_helo_name=nat.box.tld

Limitations of forced probe routing

Inconsistencies can happen when probe messages don't follow the same path as regular mail. For example, a message can be accepted when it follows the regular route while an otherwise identical probe message is rejected when it follows the forced route. The opposite can happen, too, but is less likely.