Resend

If an application loses the response to a receipt-email request, a retry can otherwise become a second receipt. Resend accepts an idempotency key for that request and retains it for 24 hours. A key based on the event and its identifier can distinguish one purchase receipt from a later refund message about the same purchase. Reusing the key for the same request within that window returns the earlier response without sending again. SMTP has a corresponding email header.

Resend prices transactional and marketing usage separately. Broadcasts go to existing contacts and do not consume the transactional allowance; inbound emails do consume that allowance. Free includes 3,000 transactional emails per month, limited to 100 per day, plus three domains and three inboxes. On paid plans, enable Transactional Overages in team settings to continue beyond the monthly quota. Extra inboxes or domains can add charges. The retry window also ends after 24 hours, so it is not a permanent record of which business events have already produced email.

Create a Resend account using Google, GitHub or an email and password. Paid subscriptions are monthly; the pricing FAQ says annual billing is not available. Broadcasts use their own contact-based plans.