Syntax checks after rewriting

No additional syntax checking is done after the rewrite rules have been applied to an address. This laxity is deliberate --- it makes it possible for rewrite rules to be used to convert addresses into formats that do not conform to RFC 822. However, this also means that configuration mistakes in the rewrite rules may result in messages leaving the MTA system with incorrect or illegal addresses.

See also:
 * Application of rewrite rules to addresses