Unable to receive emails-Exchange 2003 on SBS 2003

Hi experts,

Am having a problem on my Exchange server 2003 runnning on SBS 2003. I am unable to receive any emails from outside. The server was working fine still last week. The ISP say that emails are delivered to my server and get rejected by my server.
No changes has been done to the server since it was implemented and the internal emails and outgoing is working fine.

Appreciate if anyone could help me solve this issue.

Thanks in advance.

Nick