As a long time Apple person, and a dedicated Mac user, starting with Mavericks, I had stopped using Apple’s built in mail client.
I stopped using it for a variety of reasons, but essentially, the integration with Google mail really became, uh, shitty. Really weird, unreliable connections to the Gmail IMAP server, and finally I just gave up.
I also subscribe to a mailing list of some really cool Mac OS-X users, and around that time there was tons of traffic about how shitty Apple mail was.
I ended up first using Thunderbird (meh), and finally bought licenses to Postbox as it worked pretty well, and their support for Google’s mail services was quite good.
Finally, with a recent upgrade to El Capitan (OS-X 10.11.2) the experts allowed that Apple finally un-borked their mail client. It even handles two factor authentication properly. Kudos!
So, I am giving it a second try. And you know what? It isn’t bad. It is better than Postbox.