It turns out that the problem was on our end. Thanks for helping to narrow it down.
Currently, all of the rendered messages display the QA images with few exceptions.
The ones that still fail are:
Apple Mail 7
Lotus Notes 6.5
Lotus Notes 7
iPhone 6+ (iOS8)
Office 365 IE 11 (win)
We don’t believe this is caused by any DNS or permissions issues, so we will investigate these separately.
Thanks again for all of your help!