Postmark - Postmark is the easiest and most reliable way to be sure your important transactional emails get to the inbox. Simply & reliably parse recieved email to JSON for your webapp.
GetNotify - GetNotify is a free email tracking software solution.
SendGrid - Learn why more developers choose SendGrid for their email deliverability service to send transactional emails triggered by web apps. Get started for free
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.
JangoSMTP - JangoSMTP is a transactional email delivery service that adds tracking and authentication to email.
Mailtrack.io - The best email tracking solution I've found. Free