The message I have seen is
"Unable to add to the server Junk E-Mail Lists, you are over the size
allowed on the server. The Junk E-Mail Filter on the server will be disabled
until your Junk E-Mail Lists have been reduced to the size allowed on the
server.
Would you like to manage your Junk E-Mail Lists now?"
The answer we found was to turn on OWA (temporarily as we do not use OWA).
Enable the user in AD to use OWA. Log onto the mailbox via OWA and select
Options. Go to the Privacy and Junk E-Mail Prevention section. Unselect
Filter Junk E-Mail folder regularly to ensure that you do not miss messages
that you want to receive and Save and Close. Go back into the options and
reselect (check) the option Filter Junk E-Mail folder regularly to ensure
that you do not miss messages that you want to receive and Save and close.
This seemed to have woken up the filter so that it would work again for us.
Hope this helps you.