We are sending quite large amount of email messages to our customers every month with Postfix. Big percentage of them have their email hosted with @example.com. But every month we are struggling to deliver all the messages because of greylisting.

Example.com provided us with specific settings for every 5 minutes window to stay out of greylist:

Max. cumulative limits for every 5 minutes to domain example.com

  • 300 connections
  • 100 000 messages
  • total size of 180 MB

We are definitely not sending 100K messages so number of connections over this 5 minutes window seems to be the main issue. Could anyone help with specific Postfix settings to meet the criteria above only for connections to example.com?


These values can be edited in Postfix's main.cf -

initial_destination_concurrency parameter (default: 2) controls how many messages are initially sent to the same destination before adapting delivery concurrency. Of course, this setting is effective only as long as it does not exceed the process limit and the destination concurrency limit for the specific mail transport channel.

The default_destination_concurrency_limit parameter (default: 10) controls how many messages may be sent to the same destination simultaneously. You can override this setting for specific delivery channels (local, smtp, uucp etc.). The main.cf file recommends the following:

default_destination_concurrency_limit = 10

A destination concurrency limit of 10 for SMTP delivery seems enough to noticeably load a system without bringing it to its knees. Be careful when changing this to a much larger number.

More information here -

Postfix mail size counting

How to rate limit outgoing SMTP connections for Postfix

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