Rejecting unlisted senders in Postfix

Question

A question from a beginner Postfix admin. What is the exact difference between writing in main.cf:

smtpd_reject_unlisted_sender = yes

and

smtpd_sender_restrictions =
    reject_unlisted_sender

?

Is there a real difference in operation or is smtpd_reject_unlisted_sender just a way to enforce this restriction when some of rules listed in smtpd_sender_restrictions results in PERMIT ?

Answer 1

The first option apply to all local senders, the second permit to have a criteria like:

smtpd_sender_restrictions =
    check_sender_access hash:/etc/postfix/acl_unknown_permited
    reject_unlisted_sender

on file /etc/postfix/acl_unknown_permited

nonexistent@example.com permit

The 'permit' inside /etc/postfix/acl_unknown_permited make restriction process jump to smtpd_recipient_restriction without process reject_unlisted_sender. The right hand of file support a acl list, like:

nonexistent@example.com permit_mynetworks, reject

This create many possiblilities.