first begins to run, it figures out what your DNS domain is and
assigns the result to the $m macro ($m). The sendmail program
then processes the configuration file. This gives you the opportunity
to redefine $m. After that,
sendmail assigns the final value in
$m to the class $=m.
Unfortunately, prior to V8.10 sendmail, the
class macro $=m was not used by
sendmail, nor by any of the configuration files
produced by the m4 technique. Beginning with
V8.10, $=m is used as part of screening to allow
relaying. Note that $=m should not be used to have
mail accepted as local under a variety of domains. Instead, use the
domaintable feature (FEATURE(domaintable)).