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Postfix Documentation
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Connection cache safety mechanisms

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.

    With Postfix 2.2 the use count is limited with the smtp_connection_cache_reuse_limit configuration parameter. With Postfix 2.3 this is replaced by a time limit which is specified with the smtp_connection_reuse_time_limit parameter. In addition, 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[30840]: 19B6B2900FE:
    to=<[email protected]>, orig_to=<[email protected]>,
    relay=mail.example.com[1.2.3.4], 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 Documentation
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