class Net::IMAP::UnknownResponseError

Error raised upon an unknown response from the server.

This is different from InvalidResponseError: the response may be a valid extension response and the server may be allowed to send it in this context, but Net::IMAP either does not know how to parse it or how to handle it. This could result from enabling unknown or unhandled extensions. The connection may still be usable, but—depending on context—it may be prudent to disconnect.