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
EmailJS - Send email directly from your client-side Javascript code – no server side code required. Add static or dynamic attachments, dynamic parameters, captcha code and more. Start with our free tier!
Mailcheap - Unlimited enterprise email hosting
Mandrill - Mandrill is a new way for apps to send transactional email. It runs on the delivery infrastructure that powers MailChimp.
Context.io - Context.IO is a REST email API that makes it easy and fast to integrate your user's email data in your application