class Net::IMAP::UnknownResponseError

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