You have to configure your email client program before you can use it to retrieve your email on the host.
In general, you have to define your account name, your incoming and outgoing server names.
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 your local folders in your email client and you can no longer access your emails via Webmail. It will be very difficult to move your emails back to the email server.