The m4 nullclient
feature (FEATURE(nullclient)) causes all mail to be sent to
a central hub machine for handling. Part of what it does is to define
the mc macro MAIL_HUB:
This also causes $H to be defined as
hub. If MAIL_HUB has a value, and if
LOCAL_RELAY ($R) does not, all local email
is forwarded to hub. If LOCAL_RELAY is
defined, it takes precedence over MAIL_HUB for some mail. See Section 4.5.7 for a description of MAIL_HUB and how it
interacts with LOCAL_RELAY.
You should never use this macro directly because it might change in a
future release of sendmail. Use the MAIL_HUB
mc configuration macro instead.