Using the simple (cleartext) POP3 authentication, the entire authentication dialogue need only consist of the following ...
+OK POP server starting
user martin
+OK Password required for martin.
pass secret
+OK martin has 3 message(s) (34153 octets).
quit
+OK Pop server says bye!
Of course there are fancier scenarios using Kerberos, S/Key ... :-)
Just a thought!
Martin