Here's a double-sided scenario that has plagued me for the past few months:

I'm blazing on the keyboard at work - sending out e-mails through Microsoft Outlook like a madman - when I get a reply that reads, "Your attachment is a winmail.dat file. I'm not able to open those. Can you resend in a different format?" This does not compute. (Sorry, bad pun.) I sent her an Excel file. Hmmm. I try resending the file in PDF format. Her reply message reads, "Sorry, I'm still getting a winmail.dat file. What is that?"-OR-
I'm blazing on the laptop at home - reading e-mail messages in Thunderbird. A colleague has attached an important Word document to his message that requires my immediate attention and response. As i go to open the file, I notice that it is listed as "winmail.dat". Stubbornly, I click to open the file. I am not surprised when an error message pops up to tell me that I am unable to open this file.

Does either side of this situation sound familiar to you? If so, then you'll be happy to know that there are ways to overcome winmail.dat syndrome!

What is "winmail.dat"?

A winmail.dat file is a TNEF (transport neutral encapsulation format) attachment sent by a Microsoft e-mail application like Outlook, when someone has their Outlook set to generate e-mail messages in Rich Text Format (RTF) OR when you use Microsoft Word as your e-mail editor.

Sometimes the winmail.dat is a small file with formatting information that accompanies the original message; other times, the winmail.dat file contains the formatting information as well as the original message or other attachment files.

What can you do to make sure people don't receive winmail.dat files from your Outlook?

The most universal answer is to stop sending pretty e-mails. In other words, configure your Outlook to send messages as plain text rather than rich text format. This is an option in all versions of Outlook. Learn how to disable rich text format here. You could select to send your messages In HTML format; however, not all e-mail recipients are able to view e-mail in HTML format. All e-mail accounts are able to view plain text.

According to MozillaZine, people with Outlook 2002 (aka XP) and Outlook 2003 can take the following steps to disable sending out TNEF attachments:

On the "Tools" menu, click "Options", then click the "Mail Format" tab, and then the "Internet Format" button.
Set "When sending Outlook Rich Text messages to Internet ..." to either "Convert to HTML format" or "Convert to Plain Text format".

If you use a non-Microsoft e-mail application like Thunderbird or Lotus Notes, what can you do to convert winmail.dat files back to a viewable format?

There are actually a handful of tools that can help you to view winmail.dat files. Below is a brief list based on your computer's operating system: