Amium - A collaboration app that brings conversation to your files
Bananatag - Find out what happens to your emails after you press send! Add email tracking, attachment tracking, email scheduling and templates to your daily emails.
Dropbox Paper Mobile - Organize your team’s knowledge in a single place
GetNotify - GetNotify is a free email tracking software solution.
Dropbox Paper - A collaborative document-editing service
Mailtrack.io - The best email tracking solution I've found. Free