A sent email cannot be taken back

Every other Resend call can be run again. send-email and send-batch-emails put mail in someone's inbox, and nothing undoes that. Treat the send as the last step of a short checklist.

1. Check the sending domain

The from address must be on a domain the account has verified for sending. Run list-domains and find the domain of the from address:

• verified: send.
• pending, not_started, failed, or partly verified: do not send. Run get-domain and show the user the DNS records that are not verified yet.
• Not in the list: the account cannot send from it. Ask which verified domain to use. Do not pick one for the user.

onboarding@resend.dev sends without a domain, but only to the account owner's own address. It is for a first test, not for real recipients.

2. Prove it on a test address first

Before a real recipient, send the same message to Resend's test addresses. Remove cc and bcc, and put only test addresses in to. A real address in any recipient field gets the test email before the user confirms in step 3.

• delivered@resend.dev: delivered. Add a label for each case, for example delivered+welcome@resend.dev.
• bounced@resend.dev: a hard bounce.
• complained@resend.dev: delivered, then marked as spam.
• suppressed@resend.dev: suppressed as an address that bounced before.

Then run get-email on the result. Its status is the truth: sent means Resend accepted the message, delivered means the receiving server accepted it.

3. Confirm before a real inbox

Show the user the from, to, cc, bcc, subject, and body, and wait for a clear yes. Get this confirmation every time. Approval for an earlier send does not cover this one.

• Set an idempotencyKey (for example welcome/<user-id>), so a retry after a timeout cannot send the message twice.
• To send later, set scheduledAt. cancel-email can stop a scheduled email until it is sent, and not after.
• One message to many people who must not see each other is a batch (send-batch-emails, at most 100 per call), or a broadcast. It is not one email with a long to list.

4. Sending from code

When the user wants their application to send mail, read how the codebase already calls Resend and extend that path. The API key comes from the environment and never goes in the repo, a client bundle, or a log line. Send from the server only.

Anti-patterns

• Testing on a real person. Use delivered@resend.dev until the message is right.
• Reading sent as delivered. sent only means that Resend accepted it.
• Retrying a send without an idempotency key. A timeout does not prove that the first send failed.
• Changing the from domain so the send works. The user picks the sender, not the agent.