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If someone is feeling locked into the Microsoft ecosystem, empowering them to unshackle themselves from a large part of it is important. It makes the transition later so, so much easier when you don’t need to figure out OS-agnostic softwares across the board.
That said, if you are thinking of ripping the band-aid off all at once, I am 100% on board with it. Just not everyone is ready for it.
Thunderbird would be the go-to product for emails.