Why some of the particular images are blocked by ISPs?
Sometimes images could serve as doorways to many security problems. If the ISP blocks images by default users have to change a setting to allow images to download automatically. In such cases you can make special instructions for users on how to activate downloading, try to cooperate with ISPs that offer automatic downloading to reputable senders, or simply redesign your templates so that they show the most important content in spite of image-blocking.
