To get rid of the default domain for an ip, here is a simple way (if you have more than just the default server ip available):
From the main page in Plesk, click IP Pool in the right frame. Click the number at the far right for the ip address you'd like to see what domain is set to default. The default domain will be in bold (if one is selected).
Clicking the domain name will take you to the main page for that domain. Click Setup. At the top of the right frame you will see the IP address assigned to that domain. Drop that down and select one of the other ip address. Scroll down and click ok.
Once back at the main page for the domain, repeat the process and change it back to the previous IP address.
This process will clear the domain from being the default and cause mail.anydomainonip.com to go to the server default page