The two Mac users have combed through every available setting in Mail>Preferences as well as the settable view options in the app but can not find any discrepancy that accounts for this, nor any way to resolve the issue.
The other Mac user receives the message and can see only the body text of the message and any attachments or other formatted text in the messages shows up as a single 'winmail.dat' attachment. One of the Mac users receives the message as intended with formatted text and attachments showing up normally. A user, probably using Outlook on a PC is sending a message to two users on Macs, running the same version of Mac OSX and the same version of the Apple mail client Mail.app.
