Switched from the default win10 mail app to thunderbird about a year ago when the mail app started forcibly updating to the outlook and broke some shit on my windows installation to use a whole lot of resources. I quite liked the old mail app of the windows, but Thunderbird is quite enough of a replacement at default settings and much more customizable after fiddling. K9 has no difference than Gmail on default settings, either.
Already departed and sailing the high seas, mate.