Answers

How do I send email from a Next.js app on Vercel or Netlify?

On Vercel or Netlify, a Next.js route handler sends mail through Refresh with one fetch: a JSON POST to /api/v1/emails with a bearer API key. Code that cannot call HTTP, such as an existing Nodemailer setup, can use the SMTP relay at smtp.f5send.com on port 465 with implicit TLS.

What you get

Each API request carries a bearer API key and a JSON body, so one fetch call is enough.
An Idempotency-Key header makes a retried request return the original message id, so a retried function does not send twice.
A Test key lets you build and test the integration without delivering mail.
The SMTP relay for Nodemailer uses port 465 with implicit TLS; it has no STARTTLS and no port 587.
The SMTP username is the API key id, and the password is the key token.
Give each app its own API key, restricted to the domain of that app.
Webhook verification in a Next.js App Router handler needs the raw request body, not parsed JSON.

How it works

Verify your sending domain, then mint a SEND key restricted to that domain in API keys.
Store the token in an environment variable of the Vercel or Netlify project, never in client code.
In a route handler or server action, POST the JSON body to /api/v1/emails with an Idempotency-Key header.
For Nodemailer, set host smtp.f5send.com, port 465, the key id as user and the token as password.

Documentation

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