In using Kyoto University Mail for Student (KUMOI:st.kyoto-u.ac.jp) from the mail software or from another service, please use smtp.office365.com. Reference : Kyoto University Mail for Student (KUMOI).

In sending mails with the another SMTP server(Regardless on-campus or off-campus), there might be the case that it was not delivered judged as spoofing mails.
※Please be noted that it causes the failure that mails won't be delivered to companies or another university.

Sorry for the inconvenience and thank you for your understanding.