SBS exchange server not sending

I am looking into an issue on a SBS exchange server and I am not familiar with SBS exchange servers.
The server has been up for months and there has not been an issue. About 3 weeks ago the issue of emails not being sent out of the exchange server started to happen. Now I dug into (limited knowledge) the server and found that the server was configured to go out to an external mail server and pull in a users email (POP3). I am guessing that this could be NDR delivery issue simply because I do not know the configurations or more overly the windows configuration, or could it be something else?

Thank you in advance,

Tahoe