Who can I contact about copyright infringement of a song?

I found one of my songs is being used by someone without permission, and he's selling it at itunes in one of his albums, who can I contact to talk the details about it? I've been looking for a contact email but only found:
contactus.es@euro.apple.com
Is that ok? I wrote there, but I'm not sure if there's another email for this problem?
Thanks for any help you can give me
(Sorry if this is not the community to post this, if not, can you help me telling me where I should post it?)