As mentioned in the Quick Guide, please note that emails sent to YNU Mail (~@ynu.ac.jp and ~@ynu.jp) cannot be forwarded to external domain email addresses such as gmail (even if you set it up, it will not be forwarded).

Also, if you set up forwarding with the "Keep a copy of forwarded messages" unchecked, you will not be able to receive e-mails properly, so please do not do this.