[stcredzero]

Couldn't an "unsubscribe" functionality be written into an email server? You'd have to change the way mailboxes are stored. In particular, email threads and recipients lists would have to become explicit objects in a distributed database. Perhaps threads could be identified by a canonicalized version of the original recipients list, plus some other information?

[joesmo]

Or you could just reply to the list itself with the word 'remove' in the subject (doesn't work for all list servers). That's how most servers handled subscriptions back in the days and probably still is now.

[stcredzero]

That would work for a LISTSERV -- but this is a special kind of server that does keep the list as an explicit object. If it's a recipients list in a regular email, that would be different.