imap-login: proxy: Don't forward untagged BYE responses to the client
It will just cause confusion, especially when connections are retried.
It could end up looking like:
x login user pass
* BYE Internal error occurred. Refer to server log for more information.
* BYE Internal error occurred. Refer to server log for more information.
* BYE Internal error occurred. Refer to server log for more information.
* BYE Internal error occurred. Refer to server log for more information.
x NO [UNAVAILABLE] Account is temporarily unavailable.