External images are loaded in message despite setting 'Ask Me'

External images are loaded in message despite setting 'Ask Me'

‘Load External Images’ is set to ‘Ask Me’ on the Settings tab.

However, external images are being loaded, without my consent, in this one mail message from one specific sender. The 
'Images are not displayed - Display images below’ is not there.

The sender of the message is not in my Contacts. 

I’ve looked at the ‘Original Message' and there is definitely a call to load images in the HTML source; the images are not attached files or encoded images. (one image is a logo and the other is a 1x1 pixel gif from a tracking domain).

It’s only this one specific message.  Other messages from this sender have the 'Images are not displayed - Display images below’ prompt.

Why is this happening?