When you define MASQUERADE_AS using the m4 configuration technique, you both enable masquerading (Section 4.4.2) and assign the masquerade-as hostname to this $M macro. Note that defining $M will not enable masquerading. You must use the MASQUERADE_AS m4 configuration command to enable this service.

You should never use this macro directly because it might change in a future release of sendmail. Use the MASQUERADE_AS mc configuration macro instead.