Well, you'd have to do this all manually... For very good reasons...
Your forum would HAVE to "read in" email, even then, their mail server would have to return a specific error code which some just do not... So you could end up flagging valid messages back (away messages, challenge messages, deferred messages which state another error, etc).
even then, you'd have to have several ways to read in messages:
1. Email piping (where your server sends the email directly to the forum).
2. POP3/IMAP (where your forum would check for new messages).
#2 is the only real solution however, as some people will not have the knowlege to not bork their email configuration files and some users are on shared hosts which do not have the ability to edit the raw aliases configuration. Some people also host their email on 3rd party servers (like myself).
