This has to be the most unconventional anti-spam technique I've ever heard about.
This is intriguing. I'm going to remember this but I'm too anal about perfect A+ TLS and renewal is already fully automated these days anyway :-\
I wonder if one could setup their TLS stack to get this effect without the tradeoff...
Also, if y'all do this, it probably won't work because the spammers will start ignoring expired certs.
Yeah, even if you could find a way to deny the spammers via esoteric configuration, it'll just make them realize they forgot to turn off TLS validation anyway (which is clearly what they meant to do)