Lob - A simple API to integrate print & mail solutions into your applications.

MailClark - The Slack bot for external communications

PebblePost - PebblePost invented Programmatic Direct Mail® to transform real-time online activity into personalized, dynamically rendered direct mail.

Mailjet - All-in-one email service provider that provides maximum insight and deliverability results for both marketing and transactional emails.

Squarespace Email Campaigns - Email, but pretty.

Thunder Mail - Cheap and simple transactional emails API with Amazon SES