Question : Exchange external email suddenly stopped

We have a SBS2003 server which until some point last night worked absolutely fine. Email is delivered directly to exchange and outgoing emails are sent via SMTP connector using a dedicated server off site.

All email was working up until last night when suddenly external emails sent to us stopped arriving EXCEPT those sent using accounts on the off site server. The on site email domain is, for example, xyz.com and the off site server is 123.com, emails from 123.com arrive just fine at the Exchange server. Any other email domains do not even seem to be getting queued. There are no bounce errors sent to the sender and there is nothing in the Message Tracking Centre which might show where things are failing.

No one else has changed any settings on the server or the external server.

Given the situation I am completely stumped as to what to be looking for... I hope someone might be able to help me troubleshoot and figure this problem out.

Answer : Exchange external email suddenly stopped

Yes, the third-party server sounds like a likely culprit.