How to prevent receiving winmail.dat attachment
Unfortunately, you cannot change a setting on your end to fix it. You need to ask the person sending you to change their Outlook Mail send settings to either "Plain Text" or "HTML", nor RTF. The RTF setting is the problem.
There are several ways to do this, and it depends on which version of Outlook they are using:
2003 to 2007
In 2010, make sure the setting is: “Compose messages in this foirmat = “HTML” in File/Options/Mail
For Thunderbird, there is an add-on called LookOut that decodes the winmail.dat automtically!