When sendmail 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)).