Fail mail on purpose, before it is sent verified
Makes the engine answer chosen mail with a chosen SMTP error instead of delivering it.
In detail
Where a pause holds mail, a forced error ends it: matching messages are failed with the code we choose, and the result reaches the failure queue as though the receiver had said it. Temporary codes (4xx) therefore behave like a self-clearing throttle, and permanent ones (5xx) behave like a targeted suppression that does not need a list. The body is domain, code, extended and description, and the same targeting fields as a pause. The response echoes the row with cluster: true added — an error installed through this endpoint applies across the cluster unless scoped. It is the cleanest way to answer "stop mailing this domain and tell me it happened": the failures are visible in the result stream, they carry our own description, and the messages are not silently dropped. It is also the cleanest way to break a node, since a body without domain fails every message the node handles.