The Bat! is an email client (from Romania) specially designed for advanced email users who focus on secure email communication. It's often praised for its extreme flexibility and super powerful functions which can make you more efficiently deal with a large number of emails, so as to save you much valuable time.
Mailbird is a small, simple and user-friendly free desktop email client for Windows, written in WPF and C#. It focuses on speed and email productivity, was mainly inspired from the classic Sparrow on Mac OS, and is totally different from common email software like Thunderbird, Outlook or Windows Live Mail.