Very occasionally, we get a daemon in the admin task queue which fails forever with this message:

{
  "message": "'to' parameter is not a valid address. please check documentation"
}

This particular error is a permanent failure and reflects an invalid address format.

In the most recent case, the user has entered xxx@localhost.local. I think it's likely correct that we accept this as "probably a valid email address" and attempt to deliver mail to it (and I think it could reasonably route mail somewhere sensible under SMTP).

Ideally, this would also tie in to T13115 and immediately blocklist the address.