Cached trash in mail programs
 

Hi all,
I've noticed that both Evolution and Thunderbird keep deleted messages - even after emptying the trash - in cache folders in the .evolution and .mozilla-thunderbird directories. This seems like a wonderful way to fill up home folders with junk - is there any good reason for this, or if not, any way of deleting them without wading through hundreds of messages by hand? Some of these things contain swags of baby photos, videos, Powerpoint presentations etc. Or should I just delete a few friends? (Trouble is, I can't afford to lose the few that remain.)
If not, does anyone know of a mail app that does not do this and which can handle HTML?