Put this in the category of things that ought to be but aren’t.
An email expiry header.
My reasoning for this is simple, I subscribe to a lot of promotional stuff from various companies I’ve done, and do business with. You know things like discount codes. Most of which have an expiation date. However, there is no standard for this date. It’s often conveyed in the fine print at the bottom of the email, and the wording always varies. Some say “good thru” and a date, some say “valid until”. Moreover, the dates are almost never standardized.
All of this variation makes it hard to put together any kind of automation in my mail client to do something with these messages. With a standardized expires header — yes of course people would have to use it — I would have a standard way to deal with these messages.
E.g. maybe some promotions I want to highlight when they’re less than a week old. Others I may flat out want to delete when they’ve expired. Right now I have to manually address this on a regular basis. This is the 21st century though, why the heck do such trivial things manually?
Ergo an expires header, standardizes the date the email is no longer valid or useful so the end user’s software can easily deal with it.