Message-ID required

message.message_id_required · HTTP 400

What it means

The injected message has no Message-ID header, or its id is empty, so it was not stored. The id is what makes a retry safe: the same message sent again to the same address of the inbox is recognised and not stored twice.

What to do

Add a Message-ID header with a unique id, for example <a-unique-id@example.com>, and send the message again. Keep the same id when you retry.

Every error the API returns is a JSON problem (RFC 9457) whose code names it and whose type links to its page here. See the docs for how the API works.