It makes no difference from the programmer's side if an E-Mail gets copied from a Maildir folder to an IMAP folder or from POP3 to Maildir. Sending E-Mails through SMTP is simply done through the same mechanism. One just copies an E-Mail from a folder supporting the "receiver" protocol like IMAP, Maildir or POP3 to the SMTP folder which supports the "sender" protocol.
E-Mails are first-class objects and on-demand fetching of parts of an e-mail like the header or a particular part is supported.
Mel-base got used to implement a POP3 server, a Spam-Filter and a CLIM Mail-Reader/Composer and many small scripts to process, filter and deliver E-Mails. Mel-Base itself is Open Source and licensed under a license similar to the BSD License. The mentioned applications, while not Open Source, are available on request.
Author: Jochen Schmidt
It works in SBCL , CMUCL, OpenMCL and LispWorks with partial support for CLISP and ABCL. Porting it should be straightforward because implementation dependent parts are encapsulated in a thin compatibility layer.
As of 2020, it has an updated fork here: https://github.com/40ants/mel-base