In Settings -> Configure Kmail -> Security -> Reading there are two checkbox regarding reading "Prefer HTML to plain text" and "Allow messages to load external references from the internet". If the first one is enabled and the second not, reading an HTML email with references show a "red border box" by with I can allow Kmail to download the content (this time). But, it would be useful to add a list of "trusted" sender, so every time I read an email from that specified address, it automatically enable the load of external references.
already done in 4.10.x or 4.9.4 set it in kaddressbook and when it will indexed you will see in specific format.