edit: ah there are a few anecdotes nested in this comments already.
fedora was way ahead in performance for years. That was the only reason to go RH-alike but Fedora is _so_ flaky and systemd upstream + attendant cruft like firewalld and the default selinux policies are nightmares. Every Fedora distribution is another set of bugs and workarounds that push you towards RH enterprise..as if you hadn't been doing this for 20 years and needed RH to show you the way.
Fedora is nice if you are doing hypervsiors with QEMU/KVM. Very close to prod out the box.
When I worked at the University of Cincinnati in 2012, we had a lot of stuff running on SLES, mostly because we were running Novel's Identity Management system.
That was a while back though and I'm not sure if they're still down that route since Novell turned into Micro Focus and, I guess now, EQT.
https://penguindreams.org/blog/how-google-and-microsoft-made...
SPF, DKIM, DMARC all need to be set up. Check to see if the IP is any RBLs. Time also helps as the major MXers "learn" you are a real mail server.