I use emClient. Its paid, but its a one time fee and the client works very well event with calendar integration (gmail and outlook).

Thats really cool! Nice build :)

I use eM Client and I recommend it. It works super good, no issues with GMail calendar, Exchange server or any other weird quirks like Thunderberd (used to) have.

The disadvantages are that its paid (one time payment) and Windows only (no linux version).