Yesmail - Yesmail provides email marketing solutions that integrate mobile, web, and social channels for marketers.
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.
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
MasterBase - On demand Email Marketing software
SendGrid - Learn why more developers choose SendGrid for their email deliverability service to send transactional emails triggered by web apps. Get started for free
Thexyz - Secure premium email service with mobile sync apps for calendar, contacts, tasks, notes and FISMA compliant email archiving.