@necrevistonnezr Don't upgrade yet, I have a blog post that I will publish later today on how to update. It's a bit different than 16 to 18. That main difference is that the mysql server has to be upgraded as well.
That seemed to solve it, even if a server reboot did not solve it. I also got a mail, that this was also visible from other mail-servers (not just from my mail client), as I use DANE for certificate pinning.
I had the same issue. And the same fix. A reboot didn't work, a service restart did the trick. Thanks!