That won't work
No, it is by specific message. Once I read a particular message, I want to mark THAT message to delete itself in one week, or 10 days, or two months, or whatever, but I have to READ the message to know how long until it should be deleted. These are sometimes for "Events" like an e-mail from a friend, inviting me to something, where the e-mail message is useless after the event has taken place. I don't always make these into a calendar entry. And sometimes they are not really events, but just messages that I want to leave for a couple of months and then have go away.
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