> Basically I want the server to check if the message being sent is encrypted > with OpenGPG and either proceed or reject the message based on that criteria.
Postfix is your submission service so i think best to look that direction. If there is a header you can check for (if the header exist then allow sending) then postfix has header checks regexp/pcre that would be simple enough to setup. On the more complex side, postfix also supports policy servers you can write as a script (php, perl, python, etc) or you can go full on milter. http://www.postfix.org/header_checks.5.html http://www.postfix.org/SMTPD_POLICY_README.html http://www.postfix.org/postconf.5.html#non_smtpd_milters
