[Navarr]

If this gained widespread adoption, then spam would just +string and you'd have to allow-list.

Allow-list isn't the worst, but this isn't a long-term solution in and of itself

[hoosieree]

True, but the nice thing is it that allow-lists can be handled transparently by your email client.

Never saw this +string before? Mark as spam.

Is the +string missing? Spam.

Recognize this +string from your "Sent" folder? Keep.

Signing up for something? Generate a pseudorandom +string and add it to the allow-list.