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 address part of the information with any commentary removed. For example:

user@your.domain

Once that address is determined for the $: part of the delivery agent triple, it is copied to this ${rcpt_addr} macro.

${rcpt_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 $&{rcpt_addr}, not ${rcpt_addr}).