Postal

Self-hosted Mailgun, and it actually works. Postal handles transactional email delivery at scale — SMTP, webhooks, tracking, bounce management, multi-domain — all running on your own infrastructure. If you're sending thousands of emails and Mailgun's per-message pricing is eating your margins, Postal flips the economics.

Mailtrain is the open source newsletter alternative but focuses on marketing, not transactional. Haraka is a raw SMTP server without the management UI. Sendgrid and Postmark are the polished commercial options, but you're paying per email forever.

Postal gives you click tracking, open tracking, delivery monitoring, and a comprehensive API. It handles incoming mail too, which most alternatives skip. One instance can manage multiple organizations and domains.

The catch: running your own mail server is an ops commitment. IP reputation, SPF/DKIM/DMARC setup, deliverability monitoring — Postal gives you the platform, not the expertise. If you don't understand email deliverability, your carefully crafted emails will land in spam. Also, scaling to millions of emails means serious infrastructure planning.