Relatively new SmarterMail admin here (since December 2018 but have just renewed Support & Maintenance for another year) with a single Enterprise 250 Mailbox license; we make no money from our e-mail service but provide it as part of a bundle of other services because users expect it.
I did not see Derek's response in the thread that Alex linked to as it been deleted.
There have been no recent blog articles on MAPI, no official updates in locked threads from SmarterMail staff concerning the number of missed self-imposed release dates.
For us, MAPI was always a 'nice-to-have' and it didn't really make any difference as to whether it was provided or not but we judge each of our suppliers based on the promises they make and what they actually deliver.
Our opinion of SmarterTools isn't particularly good right now.
A quick five minute post once a week from the powers-that-be at SmarterTools would certainly help those who have made plans around the release of the MAPI feature to 'manage customer expectations'.