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I thought of this last night, when cleaning out my inbox. I had a mountain of spam, of course, but also a host of old sales notices from stores I shop at. It would really be awesome if sales email would come with a header tag for expiry date, and then you could tell your email client to auto-delete them when the expiry date had passed.

Because I don't know when the sales notice arrives whether or not I'll be able to make use of it, and having to go through old email every week or so and delete them is annoying.