HTML email / E-Newsletter - build only

We are a Design Studio in London we are looking to outsource the build of our quarterly HTML E-Newsletter. We use Mail Chimp as our email marketing service, therefore you should be aware of Mail Chimp's rules, regulations and code. I have designed the email in InDesign, the artwork is pixel perfect and based on a 12 column grid. This is a given but the email must must work across all mail platforms.