What method does the Agent use? Either it keeps a history file of the Message-ID:s of the postings you've seen, or it uses the Xref: header and marks each post in the other groups as read after you've read it the first time. (Both methods are used in other newsreaders).
I gather nfilter sets up a local proxy site. The Xref: header is marked with server name, so if Agent is set up to use a new server name for the proxy, it may be ignoring the header as it doesn't match. Or it's not getting passed on.
Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)