[tonywebster]

I've seen much more aggressive emails and letters, although I think this could have been better-resolved with a phone call to attempt clear up the immediate issues (e.g. proper attribution, terms and conditions) before even bringing up the word 'legal.'

MailChimp's concerns are quite valid, and to an extent you have to enforce IP issues, but OP is right that this happens all over the internet. If you lookup 'Dropbox' on Dribbble, you'll see tons of reproductions and remixes of their UI. I see OP's PSD as something made out of respect, and something provided to the community as an educational resource.

That doesn't change MailChimp's legal rights -- but regardless of who is right and wrong, I think a lot can be accomplished among adults having collaborative conversations. I think MailChimp is asking "How can we protect our IP?" instead of "How can we protect our IP while continuing to engage the designer and developer community?"