Well, it's a special proprietary format that Microsoft uses to send formatted text, etc., from one Outlook user to another. It's not useful to anything but Outlook, and not really that useful there, either. It's sort of like "Rich Text Format" but different.

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If you send out winmail.dat attachments to mailing lists, you're likely to get a lot of angry mail back demanding you send your mail in "plain text and not html" -- to do that in Outlook, go to the address book, select the addressee (the mailing list posting address, for instance), click on that address, right-click and select "properties", then click the "Name" tab, and at the bottom you'll see a checkbox marked "send e-mail using plain text only" -- check that box, press OK, and you'll always send mail out to that address in plain text (you can over-ride it, if you need to, by going to the "Format" menu in a new-message form, and selecting "Rich Text (HTML)" instead of "plain text").

How to turn off winmail.dat attachments

In MS Outlook:
On the Tools menu, click Options, and then click the Mail Format (or "Send") tab.
In the Send in this message format list, select Plain Text, and then click OK.

from our friends @ www.annoyances.org....

Stop Exchange from Attaching WINMAIL.DAT Last Modified: 1/6/96

A good way to piss people off on the Internet is to repeatedly fill their mailbox with the useless WINMAIL.DAT attachments that Microsoft Exchange insists on including. Since Exchange supports rich-text email (bold, italic, multiple fonts, etc.), and Internet email doesn't, any email sent from Exchange to a non-Exchange mail reader will contain an Attachment called WINMAIL.DAT. If you use Exchange, you won't see this file, and the message will retain its formatting. However, it can be confusing for those who don't use Exchange (the majority of the Internet population), and have no use for this file. Here's how to turn it off:

Step #1:
Double-click on the Mail and Fax icon in Control Panel.
Click on the Services tab, and select Internet Mail from the list.
If Internet Mail is not listed, click Add to add this service.
Click Properties, and then Message Format.
Turn off the option that reads Use MIME when sending messages.
Click OK and then OK again.

Step #2:
Double-click on the name of each recipient in your Address Book.
Turn off the option that reads Always send to this recipient in Microsoft
rich-text format.
This option needs to be set for each recipient of a message - if even one
has this turned on, all recipients will still get the attachment.