The main description of the plugin makes it sound like Gmail is an available option, only it (and several others) are NOT unless you buy the premium version. If you're going to offer versions, at least be up front about who can have what.

Great plugin! Works absolutely fine and without all the buy me advertisement that comes along with most free versions. This free version is glitch-free, does not advertise itself (doesn't seem to need to do that) and simply works! If you need to do more than basic functions, then yeah, by all means, purchase their paid version. It will be worth it.

Providers like O365 are behind the paywall, so it's just another plugin that does the same.