You kept copies of both your e-mail and his response, yes?
Those are helpful in establishing proof that you gave written notice of intent to vacate and that your landlord received it.
How many days written notice does your lease require? Colorado does not seem to have a state-mandated notice period, but rather relies on the terms set forth in the lease agreement.
Contact the Community Housing Services office for more specific advice for your situation. It sounds like your landlord is trying to be a weasel, and those sorts usually need a kick in the pants from someone in an official capacity to get them to do the right thing.

