I've found Airmail 2 has improved massively, and I use it as my primary email client on my mac, but to be honest it's still behind where Sparrow was IMO. There

I've found Airmail 2 has improved massively, and I use it as my primary email client on my mac, but to be honest it's still behind where Sparrow was IMO. There are a number of interface gripes I have, and it sometimes consumes about 1.5GB of RAM, but overall it's the best OSX email client I've used (that's being actively maintained).