This article describes how you can create a domain-specific spam policy. At first, you need to add a domain group, then add a spam policy.
To add a domain group:
To add a spam policy:
This will put the quarantined messages into the end-user digest. Save changes by selecting Add Entry.
Go to the Filtering tab. Select the spam policy that you just created from the Spam Policy drop-down menu and select Save Changes.
Verify that the group shows the correct spam policy.
Now this domain has its own spam policy.