If something doesn't work, I suggest you ``telnet localhost 110'' and first try the pop3 protocol manually (main commands are: USER username, ASS password, LOGOUT). You should see no errors and if the user is authenticated it should be added to the specified database.
You should then try to send an email with postfix to some remote user, like abc@hotmail.com and check the logs for errors.
If you can't find the error, strace could be your best friend. Good Luck.