- Free is preferable; well-maintained is a must. Nothing that stopped development in 2013, for example.
- Something directly from Google designed for this would be excellent.
- I'm running 10.11.6 El Capitan.
- I don't care if it only works with gmail and no other webmail services; that's all I'm going to use it for.
I prefer the gmail web interface, but most of my interaction with email is done offline, so I can't really escape using Mac Mail. I'd like to change that.