Inbox constraints error code

[rhiaro]

Your inbox enforces constraints about the type of data that can be sent, but responds with a 500 when the constraint is not met by the sender. You should really return a 4xx like 400 Bad Request or maybe 403 Forbidden, or feasibly 418 I'm a Teapot.

(5xx means the server messed up, 4xx means the client messed up, and the latter is true in this case).