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.
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
