Just a heads up, the documentation says we support 2012 R2 and up. SmarterMail has the technical capability to run on 2008 R2 as well, as in the installer will let you install and it will run.
We are no longer actively fixing bugs in 15 and 16. There are still cases occasionally in tickets that we'll do a fix but these are generally tickets that came in before the release. I doubt we'll come out with any more updates minus a minor to cover the fixes that we've done in tickets and such.