Trouble shoooting incomming SMTP email

I’m having a problem with incomming SMTP mail arriving at a Exchange 5.5 server. The Internet Mail Service has been configured right and has been working fine for months. Sudenly, it’s stopped receiving incomming internet email. Outgoing email worksjust fine and so does intra-office email through exchange is also working.

What is the best way to track what’s goign on with the incomming SMTP mail?