If your ISP needs you to use SMTP authentication when sending mail to them, then you just need to tell VPOP3 to use authentication when sending.

Click the Mail Connectors button at the top of the page, then click on the relevant Mail Sender on the left of the screen.

In the Settings tab, check the box called This server requires SMTP authentication and enter the appropriate username & password in the SMTP Username and SMTP Password boxes (these values are given to you by your ISP)

The Authentication Method can usually be left at Automatic for VPOP3 to choose the most secure authentication method supported by both the ISP and VPOP3.