On the old forum by default your profile was set to display your email address, also many people had their MSN on there.

There is software available that trawls through websites, building database of publically available information such as this.

So your email address (or msn address if thats the same) may well have been extracted from this forum, but it wouldnt have been sold by IB I suspect, as they would sooner market to you directly, its better business sense really as the moment they release your information to one company, that means that company could potentially do the same, even if they arent meant to.

On the new forum software, by default your email address isnt shown, so it shouldnt be an issue anymore, even on the old site though, frankly if you didnt want spam emails you shouldnt have had your email address public TBH!