lately I'm writing GMua for educational purposes and for evaluating Vala, whose purpose is to simplify writing Mail User Agents or simple scripts, ala Java Mail.
It currently parses IMAP (not yet complete) and has a graphical interface called Gutt (yes, inspired by Mutt) for testing.
In order to parse MIME parts with base64 or quopri content-transfer-encoding I chose to implement a couple of GConverter (will use GMime a day, maybe when they switch to gio, not yet needed) in Vala that you can find here:
I'm pretty sure there are bugs in these converters, by the way I wanted to share them :)