Default config: reject on too many bad RCPT

Default config: reject on too many bad RCPT

An example exploit failed against my system, because I had this sanity guard in
place; it's not a real security fix since a careful attacker could find enough
valid recipients to hit that problem, but it highlights that this is a useful
enough pattern that we should encourage its wider use.

(cherry picked from commit 2a636a39fff29b7c3da1798767a510dfed982a62)