I would like to second the "MailWasher" recommendation. It is now available through Firetrust:
http://www.firetrust.com/You don't accidentally delete valid e-mail, but you don't have to download anything either. You can view the subject lines and senders, which is usually sufficient for knowing which messages to delete. Any questionable messages can be previewed, again without downloading. Then you decide which messages to delete, bounce (send a fake "no such e-mail address" message to), and/or blacklist. Then hit "Process Mail". MailWasher will get rid of everything you don't want. When it's done, it will open up your e-mail utility, and you can download only what's left.
I love it; it's been a real life-saver for me.
Eliz.