By default postfix is setup to use error code 450 4.1.1 when a user is not on the server. This gets retried a few times before the user is notified. I would like to configure it to give a plain 550 5.1.0 Address rejected message so that it is bounced back immediately. This way the user knows they either fat fingered the email address or it simply does not exist on the system. I have looked at changing in the /etc/postfix/main.cf unverified_recipient_reject_code = 550 unknown_virtual_mailbox_reject_code = 550 unknown_address_reject_code = 550 unverified_sender_reject_code = 550 But they still get rejected as 450 4.1.1 I realize that it helps spammers determine who is real and who is not but to delay a user hours to know their email did not send is a poor option as well. Any ideas?