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.
Yesmail - Yesmail provides email marketing solutions that integrate mobile, web, and social channels for marketers.
SendGrid - Learn why more developers choose SendGrid for their email deliverability service to send transactional emails triggered by web apps. Get started for free
Mailgun - A set of powerful APIs that enable you to send, receive and track email from your app effortlessly whether you use Python, Ruby, PHP, C#, Node.js or Java.
JangoSMTP - JangoSMTP is a transactional email delivery service that adds tracking and authentication to email.
MasterBase - On demand Email Marketing software