A few users have complained about a problem in OS X 10.6 where Mail will display the text "Missing plug-in" in place of embedded images, both on received mail and on sent messages. This may have occurred after installing a software update for Snow Leopard, but seems to be isolated to OS X 10.6 so far.
This problem should only be affecting the affected client e-mail program, so any mail recipients should still be receiving their mail with properly embedded images. Therefore, if you are experiencing this problem you can still compose e-mails, its just that once they're processed by Mail and appear in the Outbox or Sent Items folders (or are received and in the Inbox folder), they may appear with this message.
It appears that the problem is associated with Java settings, and a number of people have fixed the problem by resetting their Java preferences. To do this, open the "Java Preferences" utility in the /Applications/Utilities/ folder, and under the "General" tab click "Restore Defaults" for the "Java Applet Plugin" section (though you can also reset the preferences for Java Applications as well).
If this does not work, you can try reapplying the latest "Combo" updater for your version of OS X, which has helped one or two other people. To do this, download the updater from Apple's Support site (search for "10.6 Combo") and then boot into Safe Mode by holding the "Shift" key at start-up (this will ensure a minimal boot environment for installing). When this is done, open Disk Utility and run a permissions fix on the hard drive (don't worry about any errors you may see) and then mount the Combo updater disk image and install it.
In addition to reapplying the combo updater and resetting Java preferences, you might also try resetting Mail's preferences by removing the file "com.apple.Mail.plist" from the /username/Library/Preferences/ folder. Doing this will require you to enter account information again; however, it may help fix random settings errors that may contribute to the problem. Lastly, try rebuilding your mailboxes by selecting them and choosing "Rebuild" from the "Mailboxes" menu.
It is possible that this could be the result of a conflict between Mail and third-party Mail plug-ins. Go to the /username/Library/Mail/ folder and see if a folder called "Bundles" is present. If so, open it and check out what plug-ins may be listed. These could be anything from junk mail filters to toolbar add-ons and GUI enhancers like "Widemail." Try uninstalling any unused plug-ins, or temporarily uninstalling even those you do use to see if they are contributing to the issue at hand.