Reply to a received email (the agent answers)

Sends a plain-text reply FROM the mailbox that received the message TO its Reply-To/From, threaded (In-Reply-To/References) and recorded in the conversation. Counts one quota unit and consumes warm-up like any outbound mail. Loop guard: the reply carries Auto-Submitted: auto-replied; replying to automated mail (Auto-Submitted, Precedence bulk/list/junk, null sender) is refused with 422 auto_submitted; at most 5 replies per message and 10 automated replies per conversation per hour (429). A message held by the agent-safety verdict answers 409 message_quarantined until a human releases it — the reply is never the tool that releases. When the message went to several mailboxes and only some copies are held (partial), the reply goes out from the first mailbox whose copy was delivered (or the one in from). Sending policies apply to replies as to /v1/send for the two actions that decide WHETHER mail leaves: recusar answers 422 policy_refused, and reter_para_aprovacao turns the reply into a draft awaiting a person (status held, requestedBy: policy:<name>) — that is how a key marked «require human approval» holds every reply of an agent. A key in sandbox accepts and freezes the reply without delivering it (status sandbox), even when hold or an approval policy is requested; no approval link is created. Sandbox on this endpoint comes from the key profile, not a request field or header.