Can you telnet the mail server on port 587? As I say he'll need to connect with smtp authentication.
If you can't telnet to it, then check whether postfix is listening : lsof -i tcp:587
If it is then you've probably firewalled everyone out from it, if it isn't then there's a panel problem.
And yes this is a solution to your problem, this is what we use, in fact I'd prefer it if everyone connected through port 587
Paul