Telnet is a fantastic troubleshooting tool that has not lost it's place in the support world.You can test a connection to your mailserver by using telnet. To test if you can connect to SMTP (outbound) port type;

telnet mydomain.com 25

We'll try gmails mail server;

-bash-3.00# telnet gmail-smtp-in.l.google.com 25
Trying 209.85.199.27...
Connected to gmail-smtp-in.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP g1si6939040rvb

Now type 'ehlo' (w/o the quotes) to make sure the mail server is functioning properly.

ehlo
250-mx.google.com at your service, [66.249.19.10]
250-SIZE 28311552
250-8BITMIME
250 ENHANCEDSTATUSCODES

To break out of a telnet connection press 'CNTRL ]' (the control key plus the right bracket) then type quit;

^]
telnet> quit
Connection closed.

Now to test your POP3 (incoming) mail server;

^]Quit
-bash-3.00# telnet mydomain.com 110
Trying xxx.xxx.xxx.xxx...
Connected to mydomain.com.
Escape character is '^]'.
+OK Hello there.
USER sentinal
+OK Password required.
PASS (H@Lo-3)
+OK logged in.
LIST
+OK POP3 clients that break here, they violate STD53.
1 1120
.

The line "1 1120" means that I have one message. To view the message type 'RETR 1'. You can also delete a message by typing 'DELE 1'. Type 'QUIT' to exit.