Question : Exchange 2003 (SBS) not sending or receiving.

I can telnet to port 25 from the outside, but no mail is coming or going. There are no errors in any of the event logs, and no errors when sending, just nothingness!
I don't even know where to start with this one, was working fine for years. No topology changes..

Answer : Exchange 2003 (SBS) not sending or receiving.

9 times out of 10 when I see this it's caused by antivirus/SPAM software.

Can you remove any Antivirus/SPAM software from the server and restart.  Does the problem stop?

If so re-install the antivirus/SPAM software.