The parse 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. 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 user part of the address (the gw) for local delivery, and the whole address (the gw@wash.dc.gov) for SMTP delivery.

The use of $@ to specify the host can follow only the $# prefix part of the RHS. Note that $@ has a different use when the delivery agent is named error (see $@).

The use of $@ to specify the host part of a delivery agent triple is described in detail in Section 19.5. See also See this section for how to use this $@ to specify the port to which sendmail should connect.