We're in the process of collecting opt-ins from customers to send them offers from 3rd party companies. In my professional experience whenever I've had to send third party content to my list, I've always had control over the design and build. The third party company can send style guides and assets, but the design and build is done by the list owner. Does anyone have a written source or something I can quote to this effect?

I've stressed til I'm blue in the face that ensuring control over these aspects is our responsibility as list owners as any issues with the build or design could impact our own deliverability. Surely that's reason enough?