I am building a UX report on best practices for creating emails for marketing campaigns. One recommendation I am providing is to build your designs without images and then add them after the fact to support your subject matter. This recommendation is based on the fact that to make an inclusive design, that you should make the assumption that images will not automatically be downloaded as some email providers have conditions that may result in images not downloading without user interaction.
To put weight behind this recommendation I would like to provide some analytics such as
on average 20% of email recipients will not see images in emails without intervention.
I have been unsuccessful in finding this data or something that can support the recommendation. Does anyone have some insights on this subject?