sender_unknown

Unknown sender. Answered with 403.

What it means

The submitting login is no Mailbox Tigby knows, so there is no Identity whose rules could be applied.

What to do

Submit with the Mailbox credential GET /api/identities/{handle}/identity-key handed over.

The body

{
  "type": "https://tigby.eu/problems/sender_unknown",
  "title": "Unknown sender",
  "status": 403,
  "detail": "…what was refused, naming this request's own values",
  "code": "sender_unknown",
  "doc_url": "https://docs.tigby.eu/errors/sender_unknown",
  "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.