When you open or view a message that contains graphics or open an attachment and a message similar to this:
"The properties of the message "subject" have been changed. Do you want to save changes to this message?"
when you haven't changed anything other than read the message.
When the message is HTML format with external images, you'll get this question because the message changed when you downloaded the images.
The problem is also caused by a Word add-in making changes to the Word template, usually by adding toolbar buttons. The usual cause is Adobe Acrobat but other Word templates or add-ins may also be to blame. You may get this error when you read the message or an attachment.
To fix this problem, you need to identify the Word add-in or template that is causing the message and remove or uninstall it.