An MTA listens to port 25 and accepts connections from other MTAs. The assumption is, that these MTAs do retry.
A program accepting emails from the user, MSA, listens to a different port. The common submitting mail user agent does not automatically retry to resend a message on temporary failure, but requires interaction from the user in such cases. When aegee-milter is applied in the MSA, then the âRejection message for partial delivery and partial non-deliveryâ or "Send Non Delivery Reports" mode are the only one that can work, as they do not rely on retrying. In the first mode, the errors are communitated during the SMTP dialog to the mail user agent, in the second mode, the errors are communicated as Non Delivery Report.
If a user installs a Sieve script, that is supposed to be executed during SMTP delivery from outside, it will be surprizing for the Sieve script owner to figure out, that the Sieve script was not called for mails delivered over the MSA. To match the expectations of the users, aegee-milter shall be run also in the MSA.