From MozillaZine Knowledge Base
This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).
If you see strange strings such as <!--[if !supportEmptyParas]--> <!--[endif]--> while viewing a HTML message it’s due to proprietary HTML tags created by Microsoft Word or another Microsoft application. Outlook hides these tags so somebody mainly using Microsoft software may not be aware of the problem they’re causing.
You can avoid this problem by having the sender save the HTML file as filtered HTML in Microsoft Word. This removes the proprietary tags used by Microsoft Office programs. There are also programs you can run to strip those tags if you export the message.
