Please configure your email client as below for connecting to graduate.hku.hk:
Account name: username
Email address: email@example.com
Incoming server name: graduate.hku.hk
Outgoing SMTP server: your ISP’s SMTP server name
For the Mail Server Type, select IMAP. If you select POP3 mode, all your emails will be downloaded from the email server to the local folders of your email client and you can no longer access your emails via webmail.hku.hk. Once this setting is done, it cannot be reverted.