Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Scanbot - Scanbot is a mobile app that allows you to use your device's camera as a document scanner. The app is capable of scanning a large number of source materials, and it can handle QR codes as well.
Geary - Geary is an email application built for GNOME 3.
Scanner Pro - Scanner Pro turns your iPhone into portable scanner in your pocket.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Genius Scan - On The Go.