Please, please PLEASE Mozilla
Don't bugger Thunderbird up.
It's multiplatform, handles multiple accounts and just works. Works for three generations of my family too with little support required from yours truly, the de facto IT support.
And its got nothing to do with Microsoft, Google or Apple.
And it has a useful system of extensions.
I hate using Outlook. Have to in the day job. It works, but it's not a patch on Thunderbird.