It is possible to have rewrite rules whose application is dependent upon the channel to which a message is being enqueued. This is useful when there are two names for some host, one known to one group of hosts and one known to another. By using different channels to send mail to each group, addresses can be rewritten to refer to the host under the name known to each group.
Destination channel-specific rewriting is associated with the channel to which the message is to be dequeued and processed by, and the channel keywords rules and norules on that channel. If norules is specified on the destination channel, no channel-specific rewrite checking is done. If rules is specified on the destination channel, channel-specific rule checks are enforced. The keyword rules is the default.
Destination channel-specific rewriting is not associated with the channel matched by a given address. It depends only on the message’s envelope To: address. When a message is enqueued, its envelope To: address is first rewritten to determine to which channel the message is enqueued. During the rewriting of the envelope To: address, any $C and $Q control sequences are ignored. After the envelope To: address is rewritten and the destination channel determined, then the $C and $Q control sequences are honored, as other addresses associated with the message are rewritten.
Destination-channel-specific rewrite checking is triggered by the presence of a $C or $Q control sequence in the template part of a rule. The characters following the $C or $Q, up until either an at sign (@), percent sign (%), or subsequent $N, $M, $C, $Q, $T, or $? are interpreted as a channel name.
For example, $Qchannel causes the rule to fail if channel is not the destination. For another example, $Cchannel causes the rule to fail if channel is the destination. Multiple $Q and $C clauses may be specified. If any one of multiple $Q clauses matches, the rule succeeds. If any of multiple $C clauses matches, the rule fails.