For those who ran their own mail servers it already did, via the +something notation.

Unfortunately the industry and the Internet in general went the other way.

EDIT: Oh, you mean temporary address. Easy. You have tracker nodes and receipt nodes. You publish on all tracker nodes you know your receipt node (by temporary address) every time you generate a temporary address. So those mailing you find it on trackers and post there. On that receipt node your temporary address is associated with some secret, allowing you to retrieve your incoming mail. The easiest way is that the temporary address is a pubkey and to confirm ownership you just need to sign a request for mail, or maybe it’ll be encrypted with it and no good for anyone else. Or both.