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}).