sendmail, 4th Edition

$:

Specify address in delivery agent “triple” RHS delivery agent operator

The parse rule set (formerly rule set 0) selects a delivery agent that can handle the address specified in the workspace. The form for selecting a delivery agent looks like this:

 LHS... $#delivery_agent $@ host $: address

Three pieces of information are necessary to select a delivery agent.^[255] The $# specifies the name of the delivery agent. The $@ specifies the host part of the address (for [e-mail], the host part would be wash.dc.gov), and the $: specifies the address part (the gw for local delivery, or [e-mail] for SMTP delivery).

The use of $: to specify the address can follow only the $# prefix part of the RHS. Note that $: has a different use when the delivery agent is named error or discard (see $: on page 676).

The use of $: to specify the address part of a delivery agent triple is described in detail in The parse Rule Set 0 on page 696.

^{[255] *} But note, the local delivery agent often requires only two, and the discard delivery agent requires only one.