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
email@example.com, 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
firstname.lastname@example.org) 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.