Hi
This will help.
The person that is sending you emails, has chosen in Outlook, to use the RTF format, for emailing and or specifically for emailing you. Perhaps this is default Outlook behavior at times.
This results in the winmail.dat file, which is the RTF format, of the attachments that were intended for you.
If the user would configure Outlook, to use the HTML (web) format, for emails, then you would receive the attachments as intended. As well as anybody else.
Apple mail and other mail systems, do not understand the RTF (rich text) format, and probably have to pay Microsoft licensing fees, to be able to use this format.
The options, for setting this in Outlook, differ for different versions of Outlook, but are found generally under
Tools > Address book, for setting a specific person to , use HTML format, and not RTF format, that persons email.
Also, under "Tools" > "Options", > "Mail format", for setting that all emails, will use the HTML format, instead of the RTF format.
More info
http://support.microsoft.com/kb/278061