Tip: an easy solution is to forward the mail to gmail.
Files sent by Microsoft Outlook users can have contents received by Apple Mac recipients as an attached winmail.dat file – that they can’t read.
Many on-line forums offer possible solutions to this problem. You could ask the sender to please send mail to you as either plain text or HTML-formatted email, not RTF, but many recipients will not want to pester an important client, boss or whatever this way! There are some programs (mostly commercial software) that claim to read these files, but successful results appear not to be a sure thing. Similarly, there are web services offering to convert these winmail.dat files if you upload the file to their site (with obvious security issues).
An easy solution if you don’t receive too many emails with winmail.dat files is to simply forward the message to your gmail account, log-in to gmail and read it there. Gmail will handle the contents correctly. You can also download the converted file if you need a local copy, for example to add an event to iCal.