I used to use Gnus for work email. That has an option to expire mail, though I only turn it on for automatically generated mail rather than anything a human sent. However, even for ordinary mail, it did have the newsreader-like property that once you'd seen a message you'd never see it again unless you specifically sought it out, which is almost as good as the mail expiring.

Tagging sounds good but I think requires a certain discipline to use. For people who've got it, great. (My LJ posts are quite well tagged but I write them which is a rather critical difference form the contexts of my inbox.)