MailChimp

• MailChimp is an easy to use mailing tool 

• MailChimp has triggers that can be very dynamic 

• One limitation: a subscriber can only be sent a triggered email once, so adds a bit of inflexibility if you want to send them several different emails, each time they submit a form (in our case, an order)  

• MailChimp does not host a PHP server so if you want to implement custom PHP code, make sure your CMS (content management system) can host a PHP server.  

Free Email Marketing Service

• MailChimp is an email marketing platform used to send out mass email campaigns. You can create custom templates, target lists, and campaign styles and send them to up to 2000 users for free. MailChimp also has analytics on click-through rates, purchases resulting from the email, and overall read percentage. Import recipient data easily with a CSV.
• URL: mailchimp.com

Mailgun offers a free-tier for emailing service if your application requires that functionality.  It comes with a free dashboard to manage emails sent, opened, received, rejected, etc.  It is also scalable and is built in Heroku as an add-on.