Please: Make the SM SMTP send/receive "engine" a module that is tested 100% and upgraded as needed, and works with all current and future versions. The rest of SM can then connect to this module.
Same goes for things like Exchange/Outlook compatibility, if I may be so bold.
And while you're at it, if you want a different GUI, it should be allowed to run concurrently with the older one. I'm referring, of course, to v16 vs. v15.
Without that, you've got an "us vs. them" situation, both on the same side of the argument. Nobody can win that argument.