Try telneting to your fixed ip port 110:
telnet 123.456.789.2 110
If you get a response, it will prove that the port is not blocked.
Once connected try:
user {user-name}
pass {password}
if both of these work type 'list', and you should see a list of waiting emails to be collected. Or at least a list of filenames/message id numbers.
Failing that, what are the log files in /var/log/secure and /var/log/message showing. Try:
tail -f {log file name}
Then on a PC try to collect email. Providing the port is not blocked it will show you the results of the attempted connection.
Rob