In order to access webmail, use the following convention in the address bar at the top of your internet browser:
Make sure you replace "yourdomainname" with your actual domain name.
For example if your email address is firstname.lastname@example.org you would type https://mail.mydogdont.com:20000 into the address bar at the top of the page.
When you go to this address you will see a security warning. That is because the address needs to be https (the "s" usually indicates a secure site that requires a secure certificate), but in this case there is no secure certificate required. Go ahead and continue to the website and add the exception.