Log in to your other mailbox first, click the Settings button in the upper right corner, and confirm that all emails have been selected to enable POP in POP settings, including downloaded emails.

After logging in to QQ mailbox, click Settings at the top left, click Other Mailboxes, click Add Other Email Accounts, enter the email address, click Next, then enter the password, click Verify, enter the POP3 server address pop3.aliyun.com, and click Verify again to finish adding.