This is one of the strangest problems I've EVER worked with!!
Please pay attention now, I will explain as good as I can:
Basicly one singe user in my domain complains that some emails with file attachments is never received on his PC with Outlook 2010.
For example if I scan a doc from our Xerox scanner and send it to the user he is able to see this mail on hos smartphone, on OWA and on his home computer running Outlook 2007.
HOWEVER if you start Outlook 2010 on ANY pc in the local net and log in as this user, the mail instantly disappears from EVERYWHERE.
If Outlook 2010 is open while scanning a doc from the Xerox, you are able to see the mail for a milli second on the smartphone, and then it just disappears.
This is not bound to this computer.
If you configure a new pc with Outlook 2010 it does exactly the same!
I can guarntee that there are no 3. part spam programs installed anywhere.
Please notice, that running and old Outlook 2007 ALL mails is visible (until starting up an Outlook 2010 client, then POOOF)
The mail server is running Exchange 2010.