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.
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.
SendGrid - Learn why more developers choose SendGrid for their email deliverability service to send transactional emails triggered by web apps. Get started for free
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
Elastic Email - Elastic Email is an all-in-one email delivery platform for both marketing and transactional email.
Lob - A simple API to integrate print & mail solutions into your applications.