An estimated 95 percent of all commercial e-mail messages are sent in HTML or in a multipart (combined HTML and text) format. Most e-mail includes at least a single external image, even if it's the open-tracking image, a clear, one-pixel GIF image used to track whether an e-mail has been opened.
The primary reasons behind image-blocking features are to enable users to prohibit pornographic images from loading and to prevent spammers from knowing if users open their messages. Gmail, for example, states: "Gmail disables images sent to you to protect you from unknown senders, like spammers, who use images and links to verify that your email address is real."
|Image Blocking by Major ISPs & E-mail Clients|
|External images are blocked by default||Yes||Yes||No||No||No||Yes||Yes||No|
|User controls image-blocking settings||Yes||No||Yes||Yes||Yes||Yes||Yes||Yes|
|User clicks link to enable message's images||Yes||Yes||Yes||No||No||Yes||Yes||N/A|
|Images enabled if sender is in user's address book/buddy list||Yes||No||Yes||No||Yes||Yes||Yes||Yes|
|Images autoenabled if sender is on ISP whitelist||Yes||N/A||Yes||No||N/A||N/A||N/A||N/A|
|Alt tags displayed when images disabled||No||Yes||No||No||No||No||No||N/A|
|Preview window featured included||No||No||No||No||Yes||Yes||Yes||Yes|
|Note: SP2 = Service Pack 2 upgrade for Windows XP|
|Source: EmailLabs, 2004|