This document describes the Postfix connection cache implementation, which is available with Postfix version 2.2 and later.
See Client-side TLS connection reuse for how this connection cache is used to implement multiple deliveries per TLS-encrypted connection.
Topics covered in this document:
With SMTP connection caching, Postfix can deliver multiple messages over the same SMTP connection. By default, Postfix 2.2 reuses an SMTP connection automatically when a destination has high volume of mail in the active queue.
SMTP Connection caching is a performance feature. Whether or not it actually improves performance depends on the conditions:
SMTP Connection caching can greatly improve performance when delivering mail to a destination with multiple mail servers, because it can help Postfix to skip over a non-responding server.
Otherwise, the benefits of SMTP connection caching are minor: it eliminates the latency of the TCP handshake (SYN, SYN+ACK, ACK), plus the latency of the SMTP initial handshake (220 greeting, EHLO command, EHLO response).
SMTP Connection caching gives no gains with respect to SMTP session tear-down. The Postfix smtp(8) client normally does not wait for the server's reply to the QUIT command, and it never waits for the TCP final handshake to complete.
SMTP Connection caching introduces some overhead: the client needs to send an RSET command to find out if a connection is still usable, before it can send the next MAIL FROM command. This introduces one additional round-trip delay.
For other potential issues with SMTP connection caching, see the discussion of limitations at the end of this document.
For an overview of how Postfix delivers mail, see the Postfix architecture OVERVIEW document.
The Postfix connection cache is shared among Postfix mail delivering processes. This maximizes the opportunity to reuse an open connection. Other MTAs such as Sendmail or exim have a non-shared connection cache. Here, a connection can be reused only by the mail delivering process that creates the connection. To get the same performance improvement as with a shared connection cache, non-shared connections need to be kept open for a longer time.
The scache(8) server, introduced with Postfix version 2.2, maintains the shared connection cache. With Postfix version 2.2, only the smtp(8) client has support to access this cache.
When SMTP connection caching is enabled (see next section), the smtp(8) client does not disconnect after a mail transaction, but gives the connection to the scache(8) server which keeps the connection open for a limited amount of time.
After handing over the open connection to the scache(8) server, the smtp(8) client continues with some other mail delivery request. Meanwhile, any smtp(8) client process can ask the scache(8) server for that cached connection and reuse it for mail delivery.
The connection cache can be searched by destination domain name (the right-hand side of the recipient address) and by the IP address of the host at the other end of the connection. This allows Postfix to reuse a connection even when the remote host is mail server for domains with different names.
The Postfix smtp(8) client supports two connection caching strategies:
On-demand connection caching. This is enabled by default, and is controlled with the smtp_connection_cache_on_demand configuration parameter. When this feature is enabled, the Postfix smtp(8) client automatically saves a connection to the connection cache when a destination has a high volume of mail in the active queue.
/etc/postfix/main.cf: smtp_connection_cache_on_demand = yes
Per-destination connection caching. This is enabled by explicitly listing specific destinations with the smtp_connection_cache_destinations configuration parameter. After completing delivery to a selected destination, the Postfix smtp(8) client always saves the connection to the connection cache.
Specify a comma or white space separated list of destinations or pseudo-destinations:
if mail is sent without a relay host: a domain name (the right-hand side of an email address, without the  around a numeric IP address),
a /file/name with domain names and/or relay host names as defined above,
/etc/postfix/main.cf: smtp_connection_cache_destinations = $relayhost smtp_connection_cache_destinations = hotmail.com, ... smtp_connection_cache_destinations = static:all (not recommended)
Connection caching must be used wisely. It is anti-social to keep an unused SMTP connection open for a significant amount of time, and it is unwise to send huge numbers of messages through the same connection. In order to avoid problems with SMTP connection caching, Postfix implements the following safety mechanisms:
The Postfix scache(8) server keeps a connection open for only a limited time. The time limit is specified with the smtp_connection_cache_time_limit and with the connection_cache_ttl_limit configuration parameters. This prevents anti-social behavior.
The Postfix smtp(8) client reuses a session for only a limited number of times. This avoids triggering bugs in implementations that do not correctly handle multiple deliveries per session.
As of Postfix 2.3 connection reuse is preferably limited with the smtp_connection_reuse_time_limit parameter. In addition, Postfix 2.11 provides smtp_connection_reuse_count_limit to limit how many times a connection may be reused, but this feature is unsafe as it introduces a "fatal attractor" failure mode (when a destination has multiple inbound MTAs, the slowest inbound MTA will attract most connections from Postfix to that destination).
Postfix 2.3 logs the use count of multiply-used connections, as shown in the following example:
Nov 3 16:04:31 myname postfix/smtp: 19B6B2900FE: to=<email@example.com>, orig_to=<wietse@test>, relay=mail.example.com[18.104.22.168], conn_use=2, delay=0.22, delays=0.04/0.01/0.05/0.1, dsn=2.0.0, status=sent (250 2.0.0 Ok)
The connection cache explicitly labels each cached connection with destination domain and IP address information. A connection cache lookup succeeds only when the correct information is specified. This prevents mis-delivery of mail.
Postfix SMTP connection caching conflicts with certain applications:
With Postfix versions < 3.4, the Postfix shared connection cache cannot be used with TLS, because an open TLS connection can be reused only in the process that creates it. For this reason, the Postfix smtp(8) client historically always closed the connection after completing an attempt to deliver mail over TLS.
Postfix connection caching currently does not support multiple SASL accounts per mail server. Specifically, Postfix connection caching assumes that a SASL credential is valid for all hostnames or domain names that deliver via the same mail server IP address and TCP port, and assumes that the SASL credential does not depend on the message originator.
The scache(8) connection cache server logs statistics about the peak cache size and the cache hit rates. This information is logged every connection_cache_status_update_time seconds, when the process terminates after the maximal idle time is exceeded, or when Postfix is reloaded.
Hit rates for connection cache lookups by domain will tell you how useful connection caching is.
Connection cache lookups by network address will always fail, unless you're sending mail to different domains that share the same MX hosts.
No statistics are logged when no attempts are made to access the connection cache.