description verifiedcritical
What the server said
The receiving server's response in its own words.
In detail
The least structured and most informative field. Codes tell you a message failed; this tells you why in the receiver's language — a rate limit, a block list, a policy about the sending address. It is also what response patterns match on, which makes it the raw material for the engine's own reflexes. For Hub it is what an operator actually reads when asking "why is this domain refusing us", and worth storing whole rather than truncated.