Beginning with V8.7, the RHS of a rule used to select an error or discard delivery agent can look like this:

R...          $#error $@ dsn  $: text of error message here 
R...          $#discard $: discard

For the error delivery agent, the text following the $: is the actual error message text that will be included in bounced mail or sent back to a connecting SMTP host. For the discard delivery agent, the text following the $: is generally the literal word discard.^[13]

^[13] Actually it can be anything because the text is ignored anyway.

Use of $: to specify the error delivery agent's error message is described in detail in error. Use of $: to specify the discard delivery agent is described in discard.