I can say that SM MAPI doesn't support the "OLD" MAPI protocol, but only the "new" MAPI over HTTP protocol.
This is also the reason behind it support only Outlook v16 and newer.
However, I don't know all the details
Gabriele Maoret - Head of SysAdmins at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)