It's not all that easy to write an extension.  The threshold effort is enormous.<p>So for all practical intents and purposes, I can't define my own mail categories (tags, whatever), and I can't specify a simple script to run when something comes in, and I can't access my mail as a database for bulk processing.<p>I still use Thunderbird, of course, and have for years - but these are the things I periodically try to figure out in it, and I always end up entirely stymied.  To the point where I've also seriously considered writing my own email client, if it just exposed its guts to Python or Perl or something.