customer mail relay

They are denying legitimate service, to legitimate users, whilst
attacking a legitimate business, because they don't want to
understand anything outside of their little parochial world. Some
call that ignorance.

Uh, huh.

The real answer is putting an authentication layer into SMTP.

No kidding. I don't know what life is like on the planets where .FTN
et al exist, but here on this planet, SMTP AUTH exists, and is
available in most mail clients and MTAs. The the only major MTA that
doesn't support it is sendmail (surprise) but it'll be there in 8.10.