Send electronic registered messages (LRE), Simple Letters (LS), Avis simplifié (AS) or Mail tracé (MT) to multiple recipients.

Important notes:

• LRE messages require mTLS authentication
• Resellers must specify a valid sender customer ID
• Message body must be HTML and non-empty after sanitization
• Attachments are limited to a total of 250MB
• AS messages follow the same recipient, body, and attachment rules as LS messages (no mTLS required, identifier/identifierType optional for legal persons)

MT-specific behavior:

• No mTLS or KYC required
• At most 1 attachment
• Body text must contain between 2 and 10000 characters
• No evidence generated (no legal value, no cryptographic proof)
• If the message has an attachment, the recipient receives an email with the message body and a signed link valid for 60 days
• If the message has no attachment, the recipient receives only the body inline (no download link)
• Webhook events emitted, following the email delivery lifecycle: mt_sent (sent), mt_received (delivered to the recipient's mailbox), mt_opened (recipient opened the email or downloaded the attachment), mt_undelivered (bounce; reversible), mt_failed (send failure)
• GET /messages/{id} returns only id, status and an empty evidences array for MT

Scheduled send:

• Provide scheduledAt (ISO 8601 datetime with timezone offset) to schedule the message instead of sending it immediately. Available for all message types (LRE, LS, AS, MT).
• The message is held in SCHEDULED until the scheduled time, then sent. The legal Sent event, the Datasure timestamp and the PSCO seal are produced at the real send time, not at scheduling.
• scheduledAt must be at least 15 minutes and at most 90 days in the future.
• The response's messages[].id are the 12-digit friendly ids of each contact message. Use them with POST /messages/bulk_reschedule and POST /messages/bulk_cancel to reschedule or cancel a scheduled send.
• For LRE, the sender's KYC validity is re-checked at the real send time. If it is no longer valid, the message is not sent and an lre_failed webhook is emitted.