Mailgun and Postmark sit at opposite ends of the transactional email market. Mailgun is a flexible, price-competitive workhorse: API, SMTP, marketing features, shared or dedicated IPs, and a price that keeps falling as volume grows. Postmark is a specialist: transactional mail only, a strict reputation policy on its sending IPs, and a willingness to limit what you can send in order to keep deliverability high.
Mailgun sends from a shared IP pool by default. Your reputation is tied to other customers on the same IPs, and you can move to dedicated IPs when you need isolation, which means a warmup period and a higher price tier. Most Mailgun users stay on shared IPs and never notice a problem. Some hit one, usually after a list or content change, and the provider will not tell you which neighbor caused it.
Postmark does not run a shared pool the same way. Its IPs carry only transactional mail from customers under strict content rules, so the reputation is managed, not shared. When Postmark mail goes to spam, the cause is on your side: the list, the content, or the DNS. That is a better debugging experience, and it is the main thing you are paying for.
Mailgun fits teams that need flexibility: transactional plus light marketing, high volume at low cost, SMTP and API, custom domains, and integrations. It is one of the cheapest serious options at scale.
Postmark fits teams where a single missed email has a real cost: payment confirmations, account alerts, legal notices. Its streams separate mail types so test mail and internal mail never share reputation with real user mail. It will not send marketing campaigns, period.
Both have clean REST APIs, SMTP access, webhooks for bounces and complaints, and good documentation. Mailgun has more configuration surface, which helps when you need it and slows you down when you do not. Postmark has fewer knobs and a smaller API, and its dashboard makes reputation problems visible instead of silent.
Mailgun prices by monthly volume with a free tier, and it is cheap at high volume. Postmark prices per 10,000 emails with no free tier, so it costs more per email and buys the reputation model. Check current pricing before you commit; both change.
Pick Mailgun if you need cost and flexibility, send mixed transactional and marketing, or run high volume. Pick Postmark if you send only transactional mail and a missed inbox placement is a business incident. Either way, SPF, DKIM and DMARC on your domain decide most of what happens next.
Whichever provider you pick, the setup on your domain decides the outcome.
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