The MTA provides default error messages when rewriting and channel matching fail. The ability to change these messages can be useful under certain circumstances. For example, if someone tries to send mail to an Ethernet router box, it may be considered more informative to say something like “our routers cannot accept mail” rather than the usual “illegal host/domain specified.”
A special control sequence can be used to change the error message that is printed if the rule fails. The sequence $? is used to specify an error message. Text following the $?, up to either an at sign (@), percent sign (%), $N, $M, $Q, $C, $T, or $? is taken to be the text of the error message to print if the result of this rewrite fails to match any channel. The setting of an error message is “sticky” and lasts through the rewriting process.
A rule that contains a $? operates just like any other rule. The special case of a rule containing only a $? and nothing else receives special attention --- the rewriting process is terminated without changing the mailbox or host portions of the address and the host is looked up as-is in the channel table. This lookup is expected to fail and the error message will be returned as a result.
For example, assume the final rewrite rule in the MTA configuration file is as follows:
. $?Unrecognized address; contact email@example.com