The idea of separating parsing and serialization from the mail classes themselves.
A lot of the MIME multipart API.
Showing me this can all be done in the scripting language (no C extensions necessary).
Accessing MIME parts via a File like API (thereby allowing them to be stored on disk).
“Yoshinori K. Okuji” <okuji-at-enbug-dot-org>
Code to parse messages from strings (though since then I did it a much simpler way).
Numerous other API suggestions and improvements.
Booker Bense <bbense-at-SLAC-dot-Stanford-dot-EDU>
For prodding me to implement mbox mailbox parsing (and an initial implementation). It took me months, but I did it!