All envelope addresses (sender and recipient) are passed through the parse rule set 0 so that a delivery agent can be selected. Upon receipt of the RCPT TO: address, a delivery agent is selected by the RHS of the parse rule set 0, 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:

local

Once that delivery agent is determined for the $# part of the delivery-agent triple, it is copied to this ${rcpt_mailer} macro.

${rcpt_mailer} is transient. If it is defined in the configuration file or in the command line, that definition may be ignored by sendmail. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{rcpt_mailer}, not ${rcpt_mailer}).