I had the same issue and as I was migrating from cPanel, I was astounded that (a) Centos 7 / Postfix doesn’t support SNI and (b) that the Virtualmin install guide did not have massive warnings about it and what to do to surmount the problem.
Fortunately Ilia posted how to upgrade Postfix for Centos 7 & 8 and I can confirm that his solution works perfectly. In my case I was just starting to migrate and I dumped Centos 7 and moved my production to AlmaLinux using the git script. No more SNI woes. Ilia’s ‘howto’ is at this link: https://forum.virtualmin.com/t/how-to-get-installed-postfix-3-4-with-sni-support-on-centos-7-8/106296/3