Postal - Open source mail delivery platform, alternative to Mailgun or Sendgrid

dmarcian - Safeguard your email from abuse

Mailook - Identify & fix your email deliverability issues for free 👀

Mimecast Gateway - Mimecast Gateway is a SaaS-based email security solution that uses sophisticated, multi-layered detection engines and intelligence to protect email data and employees from malware, spam, advanced threats and zero-day attacks.

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.

Hornetsecurity - Hornetsecurity offers you comprehensive, integrated managed e-mail security services.