Since it finally seems that we are close to a working MAPI version I am testing and in fact it seems that we are able to use ...
Here is the first problem I found:
In Outlook 2016 the names of the folders are shown in English instead of being translated into the language of the system, for me Italian.
This causes great confusion, because Outlook automatically creates some folders with names in Italian and therefore also in a new account there are double folders (one with English name and one with Italian name, such as "Inbox" and "Posta in arrivo" or "Drafts" and "Bozze") which, however, unfortunately, it is not clear what they correspond to in Webmail.
Furthermore, importing a PST file also does not work properly for this same reason.
I imagine that the problem is the same for all other languages, not only for Italian.
In practice at the moment the MAPI service seems to be usable only if you use Outlook in native English.