Oddly enough I've come across something similar before, except it was with a spam filter labelling something as spam, then the server rejecting the unknown header and sending it back to the spam filter, which labelled it as spam and returned it again.

I did think about using packet sniffing/logging to analyse the flow, but unfortunately it wouldn't give me enough information, and I'd need to work out how to track the connection.

Thanks for the idea :)