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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).

2 Answers 2

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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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  • Great answer, thanks. So in my case (Postfix, Dovecot, SASL, LDA, virtual users), would it be safe to say that setting Postfix to cache TLS client sessions would be beneficial because it's sending lots of mail to the outside world?
    – Jeff
    Jul 7, 2013 at 15:58
  • Yes, here you can read: " Session caching is highly recommended, because the cost of repeatedly negotiating TLS session keys is high. Future Postfix SMTP servers may limit the number of sessions that a client is allowed to negotiate per unit time."
    – user120858
    Jul 8, 2013 at 3:37
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Postfix acts as a client when it forwards mail to another host.

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