What are some alternatives?
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
SendGrid - Learn why more developers choose SendGrid for their email deliverability service to send transactional emails triggered by web apps. Get started for free
shiftmail - Email analytics made simple. shiftmail provides a REST API and web interface for intelligence gathering within email marketing.
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.
Campaign Monitor - Email marketing software built for designers and their clients to run successful email campaigns.
Elastic Email - Elastic Email is an all-in-one email delivery platform for both marketing and transactional email.