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 contains the address
part of the information with any commentary removed. For example:
gw@wash.dc.gov
Once that address is determined for the $: part of
the triple, that address is copied to this
${mail_addr} macro.
${mail_addr} 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_addr},
not ${mail_addr}).
