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.
