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I need to limit the simultaneous outgoing connections Postfix can make.

It is possible to limit the number of connection per destination with transport_destination_concurrency_limit, but not globally (or at least I didn't understand how).

For spam reasons my ISP is limiting the number of simultaneous outgong SMTP connections, so I'd like to shape this in Postfix to maximize delivery times.

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If you check postconf | grep default_destination_concurrency_limit you can see how the various --xxxxxx--_destination_concurrency_limit are set.

The default values are: smtp_destination_.., relay_destination.. etc points to $default_destination_concurrency_limit, and the value is set to 20

If a value is set to a sepcific transport, like smtp, it will override the default value (obviously)

The next item to check/match is postconf | grep destination_recipient_limit

Depending on the value set for a specific transport, like smtp, it will cause the meaning of the value set for smtp_destination_concurrency_limit as follows:

Value = 1 smtp_concurrency_limit is referring to the same recipient.

Value > 1 smtp_concurrency_limit is referring to the same domain.

Different recipients/domains are delivered in parallel, subject to the process limits specified in master.cf.

The default process limit is 100 (if not set explicitly) , so you have to limit the smtp max processes to "your value" = max concurrent accepted by your ISP

See also postconf man page and master.cf

  • Given that my problem is not related to the domain, I basically need to change the process limit. I ended up with the same solution from another thread, once I've the chance to test it I will accept the answer – Maxxer Dec 28 '18 at 14:14
  • the line to be changed is this, right? smtp unix - - y - 5 smtp – Maxxer Jan 3 at 10:00
  • In my servers, the last position is "smtpd" and not "smtp" i.e. the process used to perform the smtp outgoing connection. Maybe it is distro specific? mine is Centos – Ingvar J Jan 7 at 12:15
  • Edit: (my misstake in previous comment.I looked at the wrong config line) Yes, this looks like the correct statement – Ingvar J Jan 7 at 12:25

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