Calling all e-mail application programmers: it would be so nice if I set an expiration date (in days following receipt of the message) for mail from specific senders.

I receive all kinds of newsletters of various associations and groups, some at my primary e-mail account and some using a Yahoo! Mail account that is used for mailing lists. When I first receive a newsletter or event announcement, I want to save it, because the event hasn't happened yet. Usually the message is stored away and then three months later I no longer need the last-minute airline fare specials from September. This kind of "newsletter manager" would make keeping my inbox clean a little bit easier.