message_not_found

No such message. Answered with 404.

What it means

The Identity's Inbox holds no message with that id. The message read is the Inbox, so a message a mail client deleted – which moves it to Trash – and a message the spam filter filed to Junk are both gone from here.

What to do

List the Mailbox with GET /api/identities/{handle}/messages and use an id from there.

The body

{
  "type": "https://tigby.eu/problems/message_not_found",
  "title": "No such message",
  "status": 404,
  "detail": "…what was refused, naming this request's own values",
  "code": "message_not_found",
  "doc_url": "https://docs.tigby.eu/errors/message_not_found",
  "agent_hint": "…the next step, for software"
}

Branch on code. The detail names this request's own Handle, id or limit and is not part of the contract.