Discussion:
smtp vs. submission processes, what's the difference?
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a***@gmail.com
2020-05-07 12:03:55 UTC
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We have had postfix working for many years on three ports: 25, 587, 2525.

These are the processes we're using in master.cf:

smtp inet n - n - - smtpd
2525 inet n - n - - smtpd
submission inet n - n - - smtpd

We are running postfix version 2.10.1

Recently, we tried a new setup that would route all ports (25,587,2525) to port 25 on our postfix servers. The intent in doing so was so that we could increase the maxproc value for smtp, and disable the 2525/submission processes, and therefore we could pool resources/connections regardless of which port our users connect on.

However, some users reported connection timeouts after we made this change.

Does smtpd behave differently when listening on 587 vs. 25? (We have STARTTLS enabled on all ports, and the advertised commands between the different ports are identical).
Victor d'Agostino
2020-05-08 15:59:18 UTC
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Post by a***@gmail.com
We have had postfix working for many years on three ports: 25, 587, 2525.
smtp inet n - n - - smtpd
2525 inet n - n - - smtpd
submission inet n - n - - smtpd
We are running postfix version 2.10.1
Recently, we tried a new setup that would route all ports (25,587,2525) to port 25 on our postfix servers. The intent in doing so was so that we could increase the maxproc value for smtp, and disable the 2525/submission processes, and therefore we could pool resources/connections regardless of which port our users connect on.
However, some users reported connection timeouts after we made this change.
Does smtpd behave differently when listening on 587 vs. 25? (We have STARTTLS enabled on all ports, and the advertised commands between the different ports are identical).
Hello

It's historical.

Submission was a SMTP service supposed to be used by MTAs only.

I think starting in plain SMTP then switching to encrypted mode with STARTTLS is not allowed on this port : You have to use SMTPS from the beginning. Maybe that's why a timeout occurs.

You can read RFC 2476 (1998) from more information, but notice this RFC is made obsolete by RFC4409.

https://tools.ietf.org/html/rfc2476
https://tools.ietf.org/html/rfc4409

Regards
- Victor d'Agostino

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