Spam traps - malicious attack?

I wonder if anyone can help with this one...  We operate a fully opt-in (compliant with PECR) email database used for customer acquisition activity.  We know 100% that all the addresses on this file have been volunteered by individual registrants - the level of transparency we offer is unquestionnable.  We are therefore suprised to see that we have got caught in a Hotmail spam trap.  Given that spam traps are meant to be benign accounts set up to identify those harvesting email addresses it is extremely frustrating to see our base getting caught in one - we do not harvest addresses and have built the file perfectly legitimately.  It would seem therefore this could be a malicious attack (deliberate sign up to our base with the spam trap address) unless anyone has any ideas / suggetions as to how we are being caught?  Anyone recommend a contact Hotmail?  The 'usual' channels to communicate with them are complete rubbish.