@tty
with cyberia's email server nullhex.com, I have used a 3rd party service called glockapps to help test broad email deliver-ability across a lot of providers -- I just looked at their site and it looks even more gross and corpo / catering to spammers than it used to ... so I wanted to check to see if it still works at all.
Yes this is a very much "hold your nose" situation, but I found this tool to be helpful.
Interestingly, today I have learned something:
Microsoft now spammfolders non-allowlisted domains instead of outright /dev/null-ing them. Hey, that's a big improvement!! difference between the possible and the implausible I suppose 🙄
If you are curious, this is what the output for our server looks like: