First, no offense but, that code is atrocious. I would highly recommend scrapping it and rebuilding from scratch because you could easily clean that up. From having all those unnecessary empty table cells to using deprecated code like the <font> tag there are a ton of issues the page presents.
To your question, what is happening now is you have a div that is hidden until you get below 600px in resolution and that is an entirely different email then when you go to the full-size. At more than 600px that div becomes hidden and your email table shows. I can't find any research on it at the moment, but I suspect that display:none may not be compatible with certain email clients as written. It may need to be inline, which negates the possibility to using it in the manner you have. For instance, gmail will ignore it because it's not in the body.
In short this is not a responsive email. This is simply an email that changes the display settings of layers based on the resolution which you will have a tough time trying to get compatible across many email clients. I'm sorry to recommend this but with the way the code is and what I'm seeing, I think you are better off just making this 600px and start over with the responsive email.