Polymail - Native email app for email productivity.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
Airmail for iOS - One of the best Mac email clients, now on iPhone
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.