Why do some HTML emails show in Outlook and some don't?

Any image that is resident on the local server will show up without incident. If the image is hosted somewhere else, you may have to give Outlook permission to show them. Some of this depends on the server administrator, who can decide what image URLs are safe.

There is a good reason for this: a widespread technique in spam emails is the use of Web bugs, which are small (1X1 pixel) GIF files that you won’t even see if they’re in a document. If you look at the HTML source, you’ll see something like

<img src=“http://skum.sucking.pig/bug.gif”>

The purpose of the Web bug is to generate a server request notifying the sender that the email has been opened. This is generally a bad thing, because it confirms that the recipient’s address is valid and he’s actually reading the mail.