Check the Inbound SMTP connector. Be sure that your server is accepting inbound connections on port 25. Check your public MX records to be sure they are accurate and available.
Can you telnet to port 25 on your external IP address
See what your MX records are point to
Then telnet <mxrecord> 25 you should see header from Mail server .
If you are trying to POP mail telnet <mxrecord> 110 for POP