X-Cloudmark-SP-Filtered: true - How do I switch this off?

[jols]

I am now finding X-Cloudmark-SP-Filtered: true in the header of email sent from our servers. I don't think we had it there before. Cloudmark is way too quick-on-the-trigger IMHO, and have often blocked email for completely bogus reasons. So I'd like to switch this one off for sure.

Question is how?

UPDATE: Nevermind, it is now apparent that this was inserted along the way by someone's ISP, specifically shaw.ca

Thanks.