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 contains the host to which to connect for delivery. For example:

your.domain

Once that host is determined for the $@ part of the delivery agent triple, it is copied to this ${rcpt_host} macro. For some local delivery agents, this ${rcpt_host} macro can be undefined (NULL).

${rcpt_host} 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 $&{rcpt_host}, not ${rcpt_host}).