Roll your own Mail.app plugin
Monday, April 10th, 2006James Eagan, the creator of the Missing Attachments plugin, has written a tutorial 
on how to make a Mail.app plugin.
He writes:
In the course of writing my email un-attachment plugin for Mail.app, I found that Apple has a capable, but entirely undocumented, plugin API. I’m providing this tutorial in the hopes that it may be useful to anyone else considering implementing a plugin for Apple Mail.
Personally, I can’t understand a word of it, but many Hawk Wings readers with Python skills will no doubt find it excellent.
A poster on the macOSXHints forums has already used the tutorial to create a plugin that does something with signatures in Mail.app (adds a random signature to each email from the UNIX program Fortune?).
