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.^[12] The $# specifies the name of the delivery agent. The $@ specifies the host part of the address (for gw@wash.dc.gov, the host part would be wash.dc.cov), and the $: specifies the address part (the gw for local delivery, or gw@wash.dc.gov for SMTP delivery).

^[12] But note, the local delivery agent often requires only two, and the discard delivery agent requires only one.

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 $:).

The use of $: to specify the address part of a delivery agent triple is described in detail in Section 19.5.