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}).
