What the heck is a winmail.dat attachment?

Being new to the Macintosh, I’m “discovering” things for the first time that are ancient history for most Mac users. That is entirely to be expected. But I must admit I totally mis-diagnosed a mysterious email attachment I received today.

At the bottom of the email was this enigmatic information:

The file was from my brother, and I was actually expecting something from him- some photos. I immediately went looking to find out what “Corel Snapfire Plus” is, discovered that it is a Windows-only application, and thought I had my solution. Obviously, thought I, winmail.dat files are some sort of proprietary image format produced by Snapfire.

I was completely wrong. Apparently, winmail.dat files are the result of using Rich Text and attachments with Windows Outlook to send email to non-Windows users. The file itself is encoded using something called “Transport Neutral Encapsulation format”- the link above explains this proprietary Microsoft format in a bit more detail. Apparently, this affliction only impacts mail sent with Outlook (the exchange client): not Outlook Express, or Windows Mail (the POP/SMTP mail clients).

There are a couple of ways of decoding winmail.dat files on the Mac: the two main choices are:

  • TNEF, which is a free utility that requires you to save the winmail.dat file then drag-n-drop it to decode
  • OMiC, which is a shareware program ($10 Euros, about $15 Canadian) that integrates with Apple’s mail application and decodes winmail.dat files on the fly

I bought OMiC, so hopefully that’s the last time that particular problem will surprise me 🙂

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