Mailspring is an email app that runs on Mac OS, Windows, and Linux. I have installed it on my Google Pixelbook. site 
It is a full featured app that works well with Gmail and other email services. However, it doesn't have the same labeling/categorization of Gmail and it seems to be a little slow.
I removed it from the Pixelbook because it seemed to constantly spike the processor even when the appwas not running.