"domain"[TAB]"smtp-gateway"[NEWLINE]
Examples:
"foo.example.com" "@axmsserver.org"
sends all mail for 'foo.example.com' through the 'axmsserver.org' SMTP server, while:
"*.dummy.net" "@relay.axmsserver.org"
sends all mail for ``*'*.dummy.net' through 'relay.axmsserver.org'.
The 'smtp-gateway' can be a complex routing also, for example:
"*.dummy.net" "@relay.axmsserver.org,@mail.nowhere.org"
sends all mail for ``*'*.dummy.net' through '@relay.axmsserver.org,@mail.nowhere.org', in this way: relay.axmsserver.org --> mail.nowhere.org --> @DESTINATION.