sendmail, 4th Edition

$:

Specify message in error or discard agent “triple” RHS delivery agent operator

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.^[256]

Use of $: to specify the error delivery agent’s error message is described in detail in error on page 720. Use of $: to specify the discard delivery agent is described in discard on page 719.

^{[256] †} Actually, it can be anything because the text is ignored anyway.