Boris Veldhuijzen van Zanten writing for The Next Web:

I know it is very polite to say thanks when I answer your question offline and face to face. But in email, when you ask me a question, I answer it and you are happy with it, there really is no need to send an email with nothing but “Thanks!” in it. Want to thank me? Keep my inbox clean. Really NEED to thank me? Send me a bottle of champagne or some flowers.

Agreed also to his first rule about the CCs, I have that in place.