email-resend: pass the Resend Idempotency-Key HTTP header

[dorgz]

Summary

@payloadcms/email-resend has no way to send the Resend Idempotency-Key HTTP header. Please add one.

Facts (3.90.2)

• sendEmail in packages/email-resend/src/index.ts sends only the Authorization and Content-Type HTTP headers.
• message.headers goes into the JSON body as email headers. Resend reads the idempotency key from the HTTP header on POST /emails, not from an email header. Source: https://resend.com/docs/dashboard/emails/idempotency-keys
• Resend keeps a key for 24 hours. A second request with the same key and the same payload does not send the email again.

Why it matters

The Payload job queue claims a job with a read and then a write, with no row lock (updateJobs in @payloadcms/drizzle). Two runs that start at the same time can claim the same job. An email task then sends the email twice. On Vercel, where the queue runs from after() and a cron, overlap is possible. An idempotency key per job makes the send safe.

Proposal

One of these:

1. Read a reserved option from the message, for example sendEmail({ ..., idempotencyKey: 'job-123' }), and send it as the Idempotency-Key HTTP header.
2. Accept a headers hook in the adapter args that returns extra HTTP headers per message.

Option 1 is the smallest change. I can open a PR if the team agrees on the shape.

[1yakub]

+1, with a concrete case. Payload 3.90.2, Postgres, the jobs queue: a transactional mail is sent from a queued task. If the Resend request times out after Resend accepted it, the task retries and the same mail can go out twice. With an Idempotency-Key built from the job id, Resend would drop the second send (it keeps keys for 24 hours).

Is this planned? If not, would a PR be welcome that adds an optional idempotency key to resendAdapter (for example, a function that gets the message and returns the key)?

[1yakub]

A short follow up on the question above: is an optional idempotency key for resendAdapter something the maintainers would take as a PR? If yes, I will open one with a getIdempotencyKey(message) option and a test. If the answer is no, that is fine too, we will keep a guard on our side.

[1yakub]

Opened it as #19274 against 3.x, so there's code to look at.

[EvilConsultant]

Has anyone else been tempted to write an email marketing platform in Payload? Because after I got all my product management in payload, I started saying "Why can't I send product marketing? Why can I feed my analytics in to this and do personalized? Why can't I just use blocks as an email designer?" and now I am halfway down a rabbit hole.