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Postfix Documentation
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Connection cache implementation

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.

Internet <--
smtp(8)
 
<->
scache(8)
 
<->
smtp(8)
 
--> Internet

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.

Postfix Documentation
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