I'm the founder of AhaSend. We've built a transactional email service specifically for developers, with a Pay-As-You-Go pricing model ($0.50 per 1,000 emails) and up to 70% volume discount for platform partners.

While there are many ESPs out there, we focused heavily on the Developer Experience to solve the pain of building and testing email flows:

* The CLI: Heavily inspired by the Stripe CLI, we built a tool that lets you develop and debug locally. You can listen to webhooks, trigger events, manage your domains, and send / list / debug messages directly from your terminal.

* True Sandbox: We offer a full-lifecycle sandbox where emails go through the entire pipeline (processing, signing, etc.) without ever leaving our servers. Rhis also triggers all the webhook events (delivered, clicked, etc.) so you can test your app's logic without spamming real inboxes.

* Granular Security: We support fine-grained API scopes (e.g., reporting-only keys, or keys scoped strictly to sending from specific domains, etc), so you don't have to store "root" API keys in your application.

I'd love to hear your thoughts!