On March 22, I discovered that SOME emails to users on my server were being returned to senders. Some were definitely coming through, but some weren't. I ended up having to disable CSF, at which time hundreds of emails started to come in.
This problem didn't happen on March 21, so it was a new issue after I hadn't changed anything. So it had to be something automated.
These IPs were in my Temp Block:
64.145.93.198
50.223.38.94
72.177.80.230
And this is the only one in my Perm Block:
50.223.176.171
But IPs that were definitely blocked were (based on the ones that came in to me after I disabled CSF):
170.10.151.103
170.10.150.241
198.2.144.27
149.72.208.120
168.245.59.112
167.89.89.106
You can see that none of the delayed emails have IPs in the same range as those blocked.
It's also notable that I use Cloudflare, and I THINK that I have it configured properly with CSF. But I hadn't changed anything with Cloudflare recently either, and disabling CSF is what seemed to "fix" the problem.
Any suggestions on what could have been blocking the emails?