If you get an email in from somebody using Outlook and the attachments are stored in a file called winmail.dat, you can use this free app to open the files
also, here's the way to get the person with Outlook on other end to set it up so it doesn't happen again:
How to Configure Outlook NOT to send Winmail.dat attachments
To Turn off Rich Text sending for messages in Microsoft Outlook
1) Click on Tools
2) Click Options, and then click the Mail Format tab.
3) In the Send in this Message Format list, select Plain Text, and then click OK.
This will set your default sending method to Plain Text, which will lose your special formatting options with fonts, colors, etc. However everyone, no matter what email program they are using, will now be able to receive your email with no problems.