Good news, do any clients use MAPI on their server?

Yes to MAPI.  I am guessing about 25 people, including our CFO.   We configure it on request, but not by default, so it is not directly comparable to a MAPI-only environment.  Nonetheless, finished the day with no tickets about email.

We operate exclusively in U.S. English, so we are not vulnerable to problems created by UTF-8 or Punycode in address strings.

Support is working with me to resolve the problem with duplicate security providers for the same server, which is a remnant of a previous bug.   Anyone who has duplicated security providers should get it resolved sooner rather than later.