Stop Clients Login Portal

My Mautic version is: 2.15.3
My PHP version is: 7.1.32-1+0~20190902.23+debian9~1.gbp9d1be7

Your problem
My problem is:
When I send emails to clients the custom domain they can access my login portal by cutting off which takes anyone to my login portal.

Any way to get past this?

These errors are showing in the log: None

Steps I have tried to fix the problem: Searched the forum