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 $h).