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Jeff
Asked: 2013-07-08 06:37:31 +0800 CST

When does Postfix act like a client?

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When does Postfix act as a client? When it sends mail to the outside world?

I'm trying to decide if I should set smtp_tls_session_cache_database.

There is a similar setting called smtpd_tls_session_cache_database (smtpd) which is beneficial when Postfix is acting like a server.

smtp_tls_session_cache_database affects the Postfix client TLS session cache and smtpd_tls_session_cache_database affects the Postfix server TLS session cache.

I also read the Postfix TLS readme, but couldn't find what I was looking for.

For what it's worth, my setup has Postfix receiving mail on port 25 (passed to Dovecot) and port 587 setup as SASL auth'd relay (also Dovecot).

postfix

2 Answers

  1. Best Answer

    Postfix acts as a client (SMTP client) when it need delivery an email using SMTP protocol, see Postfix Architecture Overview.

    When Postfix receive an email, it acts as a SMTP server (ending with 'd' of daemon, smtpd) but when it need send an email to the "outside world" it acts as a SMTP client, connecting to another SMTP server, that could be a Postfix server or not.

    For ilustration:

    smtp:RANDOM_PORT      ---->     email.server.com:25

smtpd:25              <----     client.com:RANDOM_PORT
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  2. Postfix acts as a client when it forwards mail to another host.

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