I am currently trying out a Google Chrome browser extension, called Email Tracker, that works with Gmail, Outlook / Hotmail, Google Inbox, and Yahoo Mail. Its’ main sole purpose is to alert you when your sent emails have been read. Simply install and any future emails you send are automatically tracked. No account signup is required. Email Tracker is currently carrying an excellent review rating (based on approximately 200 reviews).
A circle will appear when the tracking is enabled on the sent email and a checkmark will appear when it has been read. Works for unlimited email accounts and sent emails.