must not be used for mail filtering was created several years ago for commercial DNS providers to share information about problem domains and to offer a mechanism to prevent abusers from hopping from DNS provider to provider.

Under no circumstances should it be used by any MTA for mail filtering in any capacity whatsoever. Absolutely no vetting goes into listing and domains can get listed for internal reasons which are purely meaningless in an email context.

Over the years some big name domains have wound up in the zone for reasons which have absolutely nothing to do with email and simply for inter-provider communications between the members.

In short, using on any MTA is an act of mind-numbing recklessness. Don't do it.

Thank you for your time.