This is a problem encountered by a lot of people and it is the cause of a lot of wasted time for helpdesk personnel.
Emails sent by Outlook users in RTF (Rich Text Format) over the internet can sometimes be changed along the way. This can make the winmail.dat file, which describes the RTF formatting to the receiving client, visible as a potentially unreadable attachment.
There is a Microsoft KB article about this issue here: http://support.microsoft.com/kb/278061
More information about emails formats affecting internet email in Outlook can be found here: http://support.microsoft.com/kb/290809
And if you want to prevent winmail.dat from being sent via IMC, read this article: http://support.microsoft.com/kb/149203
And to prevent it from being sent to internet users: http://support.microsoft.com/kb/138053