The extensible, open source mail client.
Ranked in these QuestionsQuestion Ranking
Pro Open Source
You can change it, customize it, and make plugins for it.
Pro Great design
Simple, easy to use, easy to learn user interface.
Currently there are plugins for GitHub integration, Templates, and Translation, and the potential for many more to come. It's built on top of Chromium so making plugins for it will not only be easy for Chrome app and extension developers, but it means that it has the potential to use Chrome plugins, too.
Pro Dynamic, secure, private
You can use Nylas' servers, or run your own mail server on your own computer.
Pro Works with Exchange accounts
N1 is one of the few 10 Windows Desktop Clients to offer support for a business Exchange email account (the Windows Native Client is one of the few alternatives that also work). It seems that this niche is really dominated by Outlook, and few solutions are available that prioritize it.
Con Some features require a monthly subscription
Con A lot of RAM memory used
This email client uses a lot of RAM (random access memory) - actually using more than Chrome (albeit with a lot of tabs open).