Exchange NNTP problem

I've managed to setup a USENET newsfeed on my Exchange server. The pull part of the connection works perfectly but the push part keeps failing with a Event ID error of 13055 and the message reads:

Server response: 441 Can't set system "NNTP-Posting-Host" header

I've looked in the knowledge base but it only covers a posting to a moderated newsgroup which I am not doing.

Any help/ideas/pointers appreciated!!

Thanks,

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Gary Williams