Scarab, thanks for the info! (alas, I have no ideas about the TLS issues)
I actually migrated to a new server this past weekend as well, from an earlier version of SM, but used 15.7.6782 on the new machine - I decided even before the reports started coming in to hold off on .6821 for now - I don't use the anti-spam features (we use Symantec email security.cloud for in/outbound filtering), and figured the lighter load on the server would make us OK.
The migration went very well, with very few problems - I did my homework, it seems - and SM seems to be behaving, user experience and server resource-wise as well.
Besides the usual collection of smartphone users (mostly IOS), and most desktop users on webmail, we also have users on Thunderbird, Mac Mail, and Eudora(!) with POP3 (Eudora's IMAP implementation never worked right, and never will; Eudora's last version is something like 15 years old).
Ultimately, most users on this server will end up on Office 365; a few mailboxes will probably remain on SM.
As I said earlier on here somewhere, I am holding off upgrading from 15.7.6782 for awhile to see how .6821 shakes out.
Will be following this thread and others. Good luck to all of us!