Upon receipt of the MAIL FROM: address, a delivery agent is selected by the RHS of a parse rule set 0 rule, which defines a triple that contains three pieces of information for that address: $# delivery_agent $@ host $:address The $# portion of the triple specifies the delivery agent to use for delivery. For example: esmtp Once that delivery agent is determined for the $# part of the triple, it is copied to this ${mail_mailer} sendmail macro. ${mail_mailer} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail . Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{mail_mailer} , not ${mail_mailer} ). |