Well you can test each step if you know a little about the protocols--
SMTP -
http://qmail.jms1.net/test-auth.shtml (great resource for that particular item)
POP3
Code:
telnet server 110
user username (or username@domain.tld, or whatever you want to test.)
pass password
list
quit
IMAP
Code:
telnet server 143
LOGIN username password
LIST "" *
^]quit
If you want to post your configs we could likely be of more assistance, but without knowing what you have configured and how it's hard to diagnose and correct any issues that may exist.