Rewriting: rewrite rule failure

If a host/domain specification fails to match any rewrite rule and no default rule (that is, match-all rule) is present, the MTA simply uses the specification "as-is";  i.e., the original specification becomes both the new specification and the routing system. If the address has a nonsensical host/domain specification it will be detected when the routing system does not match any system name associated with any channel. This relaxed interpretation of rewrite rule failures allows isolated MTA sites that only communicate with a small number of systems to get by without any rewrite rules whatsoever.

See also:
 * A rule to match any address
 * Application of rewrite rules to addresses