In order to protect HKU users from session hijacking, your session will be expired once your IP address changed. If it always happen after you login with valid user name and password, that means you network environment cannot provide a static source IP for you to access the Internet. For HKU staff/students, you may login HKUVPN first and access the webmail. If you are not HKU staff/student, please consult your network administrator to provide you a static IP address to access the Internet.
User Guide on Remote Access via HKUVPN (Virtual Private Network):