Mailgun
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Mailgun offers an email API and SMTP relay built for developers who want to embed email directly into applications. Its Optimize suite includes spam prediction that tests your messages against filters, domain reputation monitoring across ISPs, and authentication validation for SPF, DKIM, and DMARC. The platform tracks bounces, complaints, opens, and clicks with granular event reporting.
Best for engineering teams building email into their own products without relying on a third-party marketing platform. Compared to Postmark's transactional specialization or SendGrid's ecosystem, Mailgun balances low-level API control with managed deliverability features.