Try doing a telnet session to your POP3 port. For example...
[root@falcon root]# telnet localhost 110
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
+OK <xxxxxx.xxxxxxxx@yourdomain.com>
user
you@yourdomain.com
+OK
pass your_password
+OK
quit
+OK
Connection closed by foreign host.
For the username, it should probably be the full email address, but that may depend on what howto you used when setting up qmail.
You may also want to run the command...
/var/qmail/bin/qmail-showctl
and post the output here for us to see.