Google Mail Loader
I started using Thunderbird recently, but while foolishly playing with the settings, I had it delete all mail on my account on the Math Dept's IMAP server that was older than 1 month. Fortunately they keep 2-week backups, and I was able to recover my mail.
Anyway, lesson learned, I have now set about creating a backup of that mail in a gmail account. Some guy by the name of Mark Lyon has a great little app, a Google GMail Loader that sends your IMAP emails to your gmail account. To use it, you first need to (or maybe just should?) download the mail to a local mailbox file using for example Thunderbird (the original source of my mail problems!), and then give yourself plenty of time, because the program sends the messages out one at a time, with a 2-second pause in between messages, so if you have 5 years worth of stuff, it might take a while. Anyway, if you are also in the math department, and leaving soon, this is one way to backup your emails while enjoying gmail's searchableness. (Also, since when is searchability is not a word?!)