R1:dbe479f0d9de

Don't send error/exception mail to unverified addresses

Summary:
Depends on D19017. Fixes T12491. Ref T13053. After SES threw us in the dungeon for sending mail to a spamtrap we changed outbound mail rules to stop sending to unverified addresses, except a small amount of registration mail which we can't avoid.

However, we'll still reply to random inbound messages with a helpful error, even if the sender is unverified.

Instead, only send exception mail back if we know who the sender is.

Test Plan…