Rich (HTML) Messages
Rich messages are actually complex documents formatted with HTML (Hypertext Markup Language) in exactly the same way as web pages are. They often have a layout that is based on or includes images, therefore these images are generally meant to be displayed in place. There is a special option (disabled by default) to alter this behavior when displaying incoming messages in Iconizer's Preferences. You can also set up Mail not to display remote images in HTML messages (i.e. those that are not embedded in the message and have to be loaded from a remote server). To do so choose Mail > Preferences… and click Viewing. Deselect the “Display remote images in HTML messages” checkbox.
On the other hand ordinary plain text messages can have images attached to them as well as any other kinds of files. This is where Iconizer is most useful. It ensures that either no attachments at all or only those you specify are displayed in place (i.e. directly inside the body of the message). Not all e-mail software respects this in a plain text message. Apple Mail without Iconizer usually prefers to display attachments in place, most other programs and web services prefer to display attachments as icons or lists of files. (Also see Content Disposition.)
When composing a message, you can switch between rich and plain in the Format menu. You can also set a default format in Mail preferences. Note, however, that unless you use at least two different font families, styles, sizes, or colors, Mail will send the message as plain text regardless of your current settings. If you are composing a message with images that are to be displayed in place, using rich formatting (and so embedding the images in an HTML layout) is the best way to ensure the message will be displayed consistently across different software platforms.