Answers

Which email API should I use to send transactional email from my app?

Refresh is an email API for transactional mail: POST JSON to /api/v1/emails, or use the SMTP relay on port 465 with the same key. An idempotency key stops a retry from sending twice, a Test key simulates delivery, and signed webhooks report each delivery, bounce and complaint.

What you get

POST /api/v1/emails takes one JSON message, and POST /emails/batch sends up to 100 messages in one call.
A repeated Idempotency-Key returns the original message id, and the platform does not send the message again.
The SMTP relay on port 465 uses the API key id as username and the key as password.
A Test key runs the full send contract, but the message never leaves the platform.
Refresh signs webhooks with HMAC-SHA256 and a timestamp, retries them with backoff, and lets you replay them from the console.
Transactional messages never carry the bulk unsubscribe headers.
The request and response shapes of the send, batch, read and cancel endpoints are Resend-compatible.

How it works

Verify the sending domain. Refresh generates the DKIM keys and the DNS records to publish.
Mint a SEND key restricted to the domain of the app. Start with a Test key.
POST the message to /api/v1/emails with an Idempotency-Key header, or use smtp.f5send.com on port 465.
Add a webhook endpoint for email.bounced and email.complained, and verify each signature on the raw body.

Documentation

Create a free account: 3,000 emails a month, unlimited domains, no card.