what is winmail .dat email file ?

Dealing with the winmail.dat file: the problem and the solutions
The Problem
Email users sometimes find that they receive email messages with a strange file attached, called winmail.dat. When they attempt to open this file, either it can't be opened at all, or it contains "garbage" data.

The situation causing this is that people are using several different email client programs to receive, read, and send email. The most commonly used email client programs at GPC seem to be Microsoft Outlook and Netscape (specifically the Messenger component), with a small minority of techno-geeks using Eudora. Unfortunately, Outlook does not "play nice" with the other email programs all the time. This causes problems, not for the sender of the email, but the recipient, particularly when actual files are attached to messages.

Outlook97/2000
Outlook is a rather powerful email client program with a number of features that look very attractive. Most notably, Outlook allows users to send email in a variety of formats:
as plain vanilla text with no formatting
in Rich Text Format, which allows for a limited amount of formatting, such as boldface/italic/underlined text or different fonts
formatted with the HTML formatting language so that it appears (sort of) like a web page
formatted as a Microsoft Word document.
It's these formatting options that cause the problems.
When an Outlook user composes and sends a message using either Rich Text Format or HTML Format, Outlook automagically generates a file, winmail.dat, and attaches it to the end of the message. winmail.dat contains formatting information, in a human-unreadable form, that Outlook will use on the receiving end to display this email message correctly. Unfortunately, Outlook is the ONLY email client program that can use this information! Netscape Messenger, Eudora*, and other email client programs don't understand this information.

The Solutions
If you are receiving these winmail.dat files
I assume at this point that you are not using Microsoft Outlook as your email client program, since this wouldn't be a problem if you were using it.

One solution to the problem is to visit http://www.biblet.com and download the WMDecode program found there (look about halfway down the page). This will at least allow you to decode the winmail.dat files and extract any useful attachments from them.

Other than this, there's not much you can do on your end to fix the problem, since it's not your email program generating the problem. If you just don't want to deal with the problem, the other approach is to reply to the individual who sent you the offending email and ask that they re-send the message, with the attached files, as a plain text message, not in Rich Text Format or HTML. If they don't know how to do this, you can, of course, refer them to this document!

If you are sending these winmail.dat files
If someone emails you to complain that they couldn't read your attachments, or to ask what this "winmail.dat" file is that you sent them, chances are you sent this email using Microsoft Outlook 97/2000 (or, very remotely possibly, another product using Microsoft Exchange Server). Although you are not the one having the problem, you are the one who gets to fix the problem.

You have multiple possible ways to fix the problem, depending on how you have set up your address book capabilities and whether or not you are using a mailing list or group mailing to send out the offending email. Please read the remainder of this section before you begin making changes to your settings, as there are two special situations, discussed first, that you must consider before choosing the appropriate solution.

Special Situations
If you are sending messages to a mailing list or as a group mailing
In this situation, you MUST set ALL users up so that they receive plain text email. If even one user is set up in your address book, or your default setting is to receive Rich Text Format or HTML format email, everyone will receive that format. You must either edit every address book entry for every individual on your mailing list, or change your default sending mode to plain text. Both methods are described below.

If you use an online directory (LDAP server) to look up the recipient's address
In this situation, you have no address book entry to edit, so you may either change your default sending mode to plain text or change the sending mode manually for each message.


Solution details
If the recipient is in your address book
Open up your Outlook Address Book, either by clicking on the Address Book icon or by choosing Tools->Address BookSelect the recipient's entry in your address book and open up their Properties, either by clicking on the Properties button or double- clicking on the recipient's entry.
Select the "Name" tab in the Properties dialog window.
Check the box at the bottom of the window that says "Send email using plain text only".
Click the "OK" button.

If you enter the recipient's address manually in the To: line of your email message
EACH TIME you send a message to this person, you must:
Create a new email message as you normally would, but before sending it,
Choose Format->Plain Text from the menu bar.
Now send your message.

If you want to change your default sending mode
You may change your default sending mode in Outlook, thereby sending all email messages as plain text, by doing the following:

Select Tools->Options from the Outlook menu bar.
Select the "Mail Format" tab in the dialog window.
In the first drop-down list, under the "Message Format" heading, select Plain Text
Click the "OK" button.

source : http://facstaff.gpc.edu/~jbenson/resource/winmail.htm