I've encountered this a few times. I never figured out what it was. It did not seem to be related to the server though, because it happened on different servers, and different ISPs.
It appeared to be related to Outlook and Outlook Express. The mail was fine while it was sitting on the server. I could view it there. It was when it was downloaded into Outlook or OE that the message seemed to be stripped.
It seemed to happen more often after a MS upgrade, suggesting to me that there was some slight incompatibility bug in Outlook. But, it just disappeared after a while.
A quick fix was to make sure the person sent the email as Plain Text, instead of HTML.