What is the winmail.dat file that I see when I try to open an attachment?

Context

Opening an email attachment in Outlook on Windows. Then re-opening the attachment on Mac or Windows in another email client.

Solution

When you use Microsoft Outlook to send e-mail containing attachments through the Internet, some recipients using e-mail clients other than Outlook report that the mail message includes an additional file called Winmail.dat. The file is usually very small, but cannot be opened in the mail message. The original message attachment may be included in the Winmail.dat file attachment, and is not always separate from the Winmail.dat file attachment.

To avoid including Winmail.dat, use the Plain Text mail format. To select Plain Text for all messages, follow these steps:

1. On the Tools menu, click Options and then Mail Format.
2. In Send in this message format, click to select Plain Text.

If you would like to send RTF format to certain recipients and send others Plain Text, you have two options:

• On the Format menu of the mail message, select Plain Text.

• Select Plain Text on the General page of recipient's Contact form.

Additional Information

The troublesome attachment format tha Outlook uses is called
"Transport Neutral Encapsulation Format (TNEF)".

Microsoft has a KB article that discusses this:

E-mail received from a sender using Outlook includes a winmail.dat attachment
http://support.microsoft.com/kb/278061/en-us