Other related discussions can be found here:
Marketo Removing Code for Image Scaling for Outlook at 125%dpi
Please see this thread:
Marketo Removing Code for Image Scaling for Outlook at 125%dpi
We have resolved this issue and can enable the fix for your subscription if you're interested. Please see the comments in the idea above for more details.
I just finished testing... AND IT WORKS!!!! Thanks so much, Justin! BTW, it's not just the DOCTYPE that caused the issues, but Marketo was also stripping the xmlns definitions within the <HTML> tag, just after the DOCTYPE. These critical three lines of code are now also being preserved.