Internet mail: image not visable

We receive incoming internet mail from several dealers. One of these emails does not display the image in it. What I see is the bound of the image with a little arrow in the upper-left corner, which is not red.No error message in my statusbar.

I invested this problem and found the following issue:

Viewing the page source, I inpected the link to URL of the image. In that URL, I replaced the characters =5F by the underscore and the image displayed!

So, somewhere the underscore in the URL is replaced by the characters =5F. I contacted the sender, and they don’t know this problem. I tried several setting for incoming mail: Proxy settings in the location document, conversing settings in the person document in the Domino Directory: I tried Keep in Sender’s format, prefers MIME and prefers Notes Rich Text. No one solved the problem.

Pleae help me