Plunk
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Plunk is open-source, so you can run it on your own infrastructure or use their hosted version. It sits on top of AWS SES for delivery, giving you low-cost sending with full code visibility. Both REST API and SMTP interfaces are supported, and you own the deployment.
Perfect for teams that want control and transparency, or those already deep in the AWS ecosystem. Plunk works well if you need to audit the sending code, modify the platform itself, or avoid vendor lock-in on email infrastructure.