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 default rule described below).
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.