https://postmarkapp.com/ who do only transactional emails and there's no free plan. The account isn't even complete setup until you've send an email via their API and a customer service person has reviewed your setup. It's extra steps but we found their delivery rates better than sendgrid/mailgun in the past. We have the email templates in our Ruby framework (Rails) with some inline CSS (based on https://github.com/mailgun/transactional-email-templates).
