If you notice unwanted emails (maybe from another sender), you'll know: - the sender is misusing the key - if the key had been compromised / sold / stolen, the new sender is a misbehaving party (maybe can't use the key if it's linked to the sender address or domain)

You can then revoke the key, and block the sender. The MTAs can use this information to blacklist abusers.

When the key is revoked, the sender will need to re-request a new one.

The 'solution' here is that the key is now something valuable and linked to their operations, so they won't resell this. It also becomes apparent when keys have been stolen. It would also be possible to add automatic key rotation (key update when sending an email).