I was wondering if there was a way to prohibit all outgoing messages from a specific e-mail account on my domain. It needs to be able to receive incoming messages only.

I tried playing around with the e-mail filter and user level filter, but I couldn't get anything to work properly. (Note: I'm far from proficient at this stuff, so the solution could still be in one of those two areas.)

Ideally, it would be nice if it prohibiting outgoing messages from both webmail and e-mail clients like Mac Mail. Thanks in advance!