Sends an email to the customer from the project's "Send emails from" address (Project → Emails; 422 if it is not set), with the reply signature appended and {placeholders} ({email}, {user_id}, {project_name} …) filled in. The conversation becomes open if it was unread or closed; an unassigned conversation is assigned to the sender (client keys).

Send multipart/form-data with attachments[] to attach files (up to 10, 20 MB each). To make a retry safe, pass a client_key (or an Idempotency-Key header): the same key within 15 minutes returns the first message instead of emailing again. For the demo conversation the reply is stored but nothing is sent.

{
  "conversation": {
    "status_label": "Open"
  }
}