I know this has been discussed before, however, its now January 2021. These times are a changing...
While SmarterMail is .NET its certainly possible to run .Net Applications cross-platform on Linux with some effort! While I am not an expert on .NET I do understand that Microsoft is trying to focus devs to one .Net solution to work on "everything" from Intel, and ARM CPUs plus small MicroControllers for hardware so .NET apps can on run on IOT, Android, Apple macOS and i OS, XBOX to PCs plus Servers. It even looks like Apple is going to consolidate MAC and i Device Apps over the next few years as well. I hope to be evaluating an ARM Server running VMWare or Proxmox plus Debian Virtual Machines in 2021. Its time to get those Linux skills up as I've been practicing for a couple of years now.
I have moved my VoIP Platform from Windows to Linux as the my vendor supports both which I NEVER thought I would DO! Second Generation ARM Servers with PCIe are here to run 64 Bit Linux OS. Windows will be on ARM soon as well. Microsoft is developing its own ARM CPU!
Debian is a good flavor to consider especially since Red Hat as done something interesting with CentOS. It also getting harder to use SmarterMail with ActiveSync on Android. Android 10 has dropped its stand-alone eMail Client that supports ActiveSync. Outlook on Android won't work with ActiveSync. Microsoft is looking to kill off Exchange and force everyone to their Cloud as I understand it.
So my suggestion is that SmarterMail should make a version runnable on Linux -- I know its been talked about for quite some time -- V17 is upon us!
We are also going to need an Android eMail Client. I am looking for a local Android eMail Client to integrate into Android eMail Store Format plus Contacts and Calendar so that other local Android Apps can access that Data. A Web Client won't do that! We need to be ready when Microsoft makes their left turn!
I don't know enough about Apple to comment as I just use Android.