I am encountering the error "Server denied POP3 access for the given username and password" when importing emails from one Gmail account to another

Open your personal Gmail and DLSU Gmail accounts.
Click the gear icon in the upper right. Select Settings.
Click the Forwarding and POP/MAP tab.
In the POP Download section, click Enable POP for all mail, and migrate your DLSU Gmail to your other email account again.

Note: Should the problem still occurs, open your personal and DLSU Gmail account and follow the instruction below.

Go to security.google.com
Click Sign-in & security
Click secure apps
Set the Allow less secure apps to "ON", log out and log in, then try to repeat the given procedures above.