> Optional postage for strangers: the server can answer a first contact with a 402. Configurable per mailbox; the cost of cold spamming stops being zero.

Interesting take re putting a cost on spam. Perhaps another take would be functionally implementing introductions.

> First-contact consent: an unknown sender doesn't get into the mailbox; they land in a "requests" box with their first message visible (like Signal's message requests). You accept, and the thread opens forever. A stranger can knock on your door, which is the essential property of mail, but they can't fill up your living room.

Unfortunately, this will only move the problem, and only reduce it in the case when all outside communication is ignored.