Using Thunderbird 17 with an IMAP account set to keep mail offline. Works great. Except that in order to keep my mail, Thunderbird maintains one enormous file called "INBOX". For an account of mine that's 4 years old, this one file has grown to over 30 GB! Not that I mind keeping so much data, but it's a pain to have it all in one file because when I run an incremental backup of my home folder using rsync this huge INBOX file has to be transfered each time. Way slow!
To be clear, this is the file I'm talking about: