Block risky messages with the right SMTP response

Blocking a message is not a single action, how you block it changes what happens next on both ends of the connection. MSH Postfix Milter’s block message action gives you four ways to do it, each returning a different response to Postfix during the SMTP transaction.

Discard accepts the message quietly and never delivers it, with no error sent back. Reject returns a permanent 5xx error, telling the sending server not to retry. Tempfail returns a temporary 4xx error instead, so a legitimate sender’s server will queue and retry later.

Custom codes for uncertain cases

When none of the defaults fit, Custom lets you set the exact SMTP response code, enhanced status code and message text, for example 550 with 5.7.1 and “Message rejected: Sender not authorized”. As a rule of thumb, use reject for clear policy violations, tempfail for anything that might be a transient problem, and save discard for cases where you would rather not tip off the sender at all.

To learn more visit MSH Postfix Milter website.