As a user of mac mail I started to look out for other email clients because I had trouble connecting my Google accounts. Plus Apple support was of no help. I've been using Polymail from a couple of months now. Honestly, I'm pleased with it so far. It's a great tool for sales and marketing professionals. My thoughts:

Pros :

Clean & simple UI Available for both mac & IOS Provides host of productivity tools like email tracking, send later and read later Rich Contact information like Twitter bio, photo, current job, etc Email history of a customer in one place Let's you unsend emails as well. Life Saviour.

Cons:

Email sync is slow sometimes

Note that I have 2 main requirement : 1. Open source. 2. GPG integration

Mutt on laptop, k9-mail on android.

mutt ++ : just do what you ask him to do. lighter than any other solution I've tried. you don't need your mouse to use it.

mutt -- : Not always packaged with a conf. a bit confusing at first

k9-mail ++: GPG integration (using another app). incredible tunning level. well integrated with android

K9-mail --: few UX issues