We had a few customers using Microsoft O360 who said they didn't receive our emails. After troubleshooting we discovered them in spam. For reference we were using Gsuite at a .xyz address. We checked with other customers who received our emails -- perhaps it was the domain, I attributed it to being an odd tld.

Running my own mail server. Off the top of my head, I would also say that the .xyz domain is predominantly used for spam. I remember reading on HN once that this is because it's a TLD where it's easy to register throw-away domains. But maybe that's just the effect of me remembering it more since it's unusual? Anyway, I did a quick check:

For the approximately last 12 months, the probability of a message mentioning ".xyz" in them being spam was approximately 7.4:1. In other words, for every 8 messages received that had ".xyz" somewhere in the headers or body, 7 would get classified as spam by my Bayesian spam filter and 1 would be classified as non-spam.

In fact, looking at the spam filter database, I see a huge amount of .xyz domains in there that appear obvious throwaways in the pattern wordNN.xyz where "word" is some random word and NN is a counter that counts up from 1.

I'm not saying that is the general situation. It just appears that in my corner of the net, a message coming from an .xyz is indeed a pretty strong signal that it's spam.