ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Lob - A simple API to integrate print & mail solutions into your applications.
Intelliverse Email Tracker - Intelliverse Email Tracker provides insights to increase the productivity of your emails.
PostGrid - Transform your Offline Communications. Use our fully-documented REST API to send personalized letters, checks, postcards and improve address accuracy.
Komiko - Komiko is a sales intelligence solution to drive sales, retention and expansion.
AmazingMail - Business print-mail solution