PostGrid has been great for automating our insurance mails for our client
Lob - A simple API to integrate print & mail solutions into your applications.
Mailivery - Reach the Inbox, not the spam folder.
Postalytics - Automated direct mail software that’s faster, smarter & better. Postalytics sends personalized direct mail from your CRM and analyzes delivery & response.
Sidemail - The simplest way to send transactional emails. Sidemail makes sending transactional emails trivial, it's perfect for side projects, MVPs or the next big thing.
AmazingMail - Business print-mail solution
Postal - Open source mail delivery platform, alternative to Mailgun or Sendgrid