Looking for SMTP source code

I am looking for Unix-based SMTP (the protocol as well as a user interface) 
email source codes.

If you know of a well written email source code, please send me the
the information to 

        Jisoo Geiter  
        (703) 883-6571 
        geiter(_at_)gateway(_dot_)mitre(_dot_)org

If you know of more than one, I would appreciate your assessment 
on the implementations (the good, the bad, and the ugly).


Thanks,

Jisoo Geiter