I have multiple domains.
I write a message filter for Domain A in the web panel.
If I want to use the same filter on Domain B, I have to recreate it on domain B.
Instead, it would be easier if I could write a message filter and then specify which of my domains it should run against.
I suggest that the message filter form has a list of my domains with checkboxes so I can pick which domains the filter will work with.
This user interface may not be appropriate for users who have a large number of domains. I leave it as an exercise for the reader to determine a more elegant interface;) Perhaps even just a checkbox for whether the message filter should work on all domains or a single specified domain.
It would be important for the user to still be able to see which filters run against a particular domain, without having to manually look at each filter.