Clever! Are you running your own email server, or is there a provider/relay other than Google that can route based on regexes?
The most effective thing back in the day was blocking based on TCP fingerprint, as "Windows XP" was different from "Windows 2003" IIRC (with OpenBSD PF). After that, greylisting gave me a few SPAM-free years, before it became the norm (OpenBSD spamd). Many OpenBSD users had quite a few SPAM-free years back then.
(Genuinely curious - particularly with M$)