Request body
POST JSON fields: `to` (required), optional `cc`, `bcc`, `replyTo`, and `from`, plus `subject`, `text`, and optional `priority`. Recipient fields accept arrays or delimited strings, with no more than 20 total recipients. `from`, when set, must be owned by the mailbox bound to the key.
Subject is limited to 200 characters, message text to 500,000 characters, and the JSON body to 1 MiB. The API accepts plain text only.
Example
curl --fail-with-body -X POST \
-H "Authorization: Bearer ${LUCION_MAIL_API_KEY}" \
-H "Content-Type: application/json" \
-H "Idempotency-Key: invoice-2026-10-14-001" \
--data '{"to":["recipient@example.com"],"subject":"Invoice ready","text":"Your invoice is ready."}' \
"https://lucionmail.com/api/v1/mail/messages"Accepted versus delivered
`202 Accepted` means Resend accepted the message for processing. It does not prove final delivery, inbox placement, or that the recipient read the message. Provider delivery and bounce events update status asynchronously.
Unsupported send features
API v1 does not accept attachments, HTML bodies, scheduled-send fields, arbitrary transport headers, or organization-owned sender identities. Unsupported JSON fields produce a validation error.