I have been happy with our email server being "set & forget" as I am a general IT support person, not a specialist. I would still make the recommendation to migrate to a new ClearBox appliance, except for the fact that I went through this EXACT scenario with Mac server OS. It was a nicely integrated server distro with excellent features, and while I argued -against- it at the time, I was over-ruled by the argument "Apple is a huge company with a solid track record; WTF is linux?" only to get burned bad when Apple dropped the server version. They also moved their best & brightest over to iOS, so the desktop OSX suffered a bit as well.

With Clear now getting distracted with mobile devices and other services, I am left wondering about the long-term commitment. They have to earn dollars just like all their predecessors who bailed on selling software and instead using a subscription model, with no opt-out of changes or updates.

I am seriously looking at a move to a Debian base and building a basic email server; that all I need at present.

I have played this game for DECADES now with so many folks quoting the same bullshit:

"Can't go wrong buying IBM"; then they quit the PC and OS business.
"Can't go wrong buying an Apple server"; then they bailed on server distro
"Can't go wrong buying Microsoft"; they are now pushing for subscription model and you won't be able to lock updates which can brick some custom software or older devices.
"Can't go wrong buying RHEL"; yeah until IBM bought them...

I am suspecting a pattern here...

If back in the 90's, I had built my servers on Debian, all I would have done over the years is apt-get dist-upgrade......