Tags are stored directly into a message's headers, so they are fully search- and forwardable.
You can use http://tagthe.net to auto-tag your messages via a secure connection.
Suggestion of frequently used tags. Add them without touching the keyboard.
3 different ways of searching and organizing tagged mail.
You can define an arbitrary number of external search engines (like Google or del.icio.us) where you can search for your tags.
Changelog for 1.3:
* Tag input field now is focused when opening 'edit tags' dialog.
* Suggestion button has shortkey in 'edit tags' dialog.
* Added 'remove all tags' to a messages context menu and to 'message > tag the bird' menu.