The parse rule set 0 (Section 19.5) is used to resolve the recipient address into
a triple: the delivery agent (with $#), the host
part of the address (with $@), and the recipient
address (with $:). The host part, from the
$@, is made the value of $h.
Once $h's value has been set, it
undergoes no further rule-set parsing.
$h is intended for use in the
A= equate (A=) of
delivery agent definitions. Normally, it is converted to all
lowercase before use, but that conversion can be suppressed with the
F=h delivery agent flag (F=h).
$h is also used by the
localaddr rule set 5 (Section 19.6) to process +detail
addresses (Section 12.4.4).
$h is transient. If it is defined in the
configuration file or in the command line, that definition can be
ignored by sendmail. Note that a
$& prefix is necessary when you reference this
macro in rules (that is, use $&h, not