The incident is caused by a change brought by the security maintenance on 18 Jun 2019 (Tue) evening.
Users found connection to University mailbox (i.e. Office 365) via common email clients e.g. Android / iOS Mail App, Thunderbird unstable during the above period. Their mail client repeatedly prompted “incorrect password / re-enter password”.
Users could still log in via internet browsers at https://www.cuhk.edu.hk/o365 during the above period. The problem is now fixed. Sorry for the inconvenience caused.