Possibly there is a wmf file embedded in the PowerPoint file?
wmf files are not generally used much anymore, due their big security issue a few years back (See
Windows Metafile vulnerability - Wikipedia, the free encyclopedia for more info), and many scanners today simply block wmf files as a matter of course.
We block wmf files in our hosting farm, and perhaps 2-3 users complain each year. Of those complaints, generally only one of the attachments is a legitimate (but older) business-related attachment. The other attachments most would consider as "not safe for office viewing" by any stretch.
In your case we would advise the sender to remove the wmf file from the PowerPoint file and resend.
Hope that helps,
Mark
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