block .local (invalid) emails (spam issue)

My server is getting attacked with large amounts of incoming spam emails (thousands an hour!), which delays delivery of normal valid emails, and causes all sorts of issues.

All of these spam messages show the same invalid sender address in the Mail Queue: <postmaster@3-it.local> (see below)

I added 3-it.local to the Black List at the Server-Wide Mail Settings page, but that won't stop these emails from coming in. Adding postmaster@3-it.local to the...

block .local (invalid) emails (spam issue)