El Capitan comes with a mail application that is pretty good. I like Thunderbird because of the customization. I have tried all the other clients and finally bought AirMail based on a lot of recommendations. I did like it at the start because it auto configured my three email accounts without any tweaking. And I do like the color theme that associates with an account and a emblem for each sender.
What I was surprised was it doesn’t support encryption. Everyone supports encryption. If you use it, you should wait until they build it into the product. Now it takes you to either Github where there is no code for GPG or to some other site that could be anything.