A rule to match bang-style addresses
If the MTA tries to rewrite an address of the form B!A and fails, it tries one extra rule before falling through and treating this address form as B!A@localhost. This extra rule is the bang-style rule. The pattern is $!. The pattern never changes. This rule is only activated when a local part containing an exclamation point has failed to rewrite any other way (including the match-all rule).
The bang-style rule can be used to force UUCP style addresses to be routed to a system with comprehensive knowledge of UUCP systems and routing.