An Outlook user asked: “While pasting images to a plain text email, a dialog box will pop up:

Microsoft office outlook compatibility checker: some of the features in this document aren't supported by plain text e-mail.
Summary
<feature> will be lost

It includes a check box for Don't show this dialog box again. I accidentally clicked on the check box. How do I get that dialog box back?”

Outlook reads this key when it loads and updates this key when it exits, so you'll need to close Outlook before deleting the value.

This setting is stored in the following registry key: HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Word\Data
Value: SettingsWordMail

To show other dismissed dialogs, see
http://www.slipstick.com/outlook/showdialog.htm