What is winmail.dat

I am getting a lot of complaints from my freinds and relative that, they are receiving a attachment in my mails called winmail.dat file. As i had never attached any file in the mail. I just send a clear messages to my freinds. I am not aware that how these attachment's are created automatically in my emails. I am using Outlook Express to work with my mails. What is the cause for this and how to get rid of it. Help.

Winmail.dat is an outlook express mail's error. Basically Outlook Express sends your mail in RTF format. It not a popular format for all other users. And when you are sending a email you are using the bold, italic and other formatting features. The way it works is, when outlook express sends a message using the RTF format of a formatted message, it includes the formatting commands in the winmail.dat file. It is created automatically as the receiver's email client is unable to read it. To clear this out do this :

1. Open your Outlook Express > Go to Tools > Option
2. Go to Mail Format Tab > Compose. In this window select either HTML or Plain Text.
3. Now click on Internet Format.
4. Make sure either Convert to Plain Text format or Convert to HTML format is selected under When sending Outlook Rich Text messages to Internet recipients, use this format.
5. Click OK.

You can stop this error by selecting the Plain Text option from the Mail format in Tools > Option. If only a few number of people are getting this attachment then you have to configure it from the address book.
Try this :
Launch Outlook Express. Click Address Book
In the Show names, select Personal Address Book.
Select on the name to whom you want to send mail. And now click on properties on the File Menu.
In General Tab go to SMTP and uncheck Always send to this recipient in Microsoft Exchange rich text format check box.
Click OK
Relaunch your application.

It is not a matter of worry. Winmail.dat file is an a simple file created by outlook express if your receiver's email client is not able to read the format of your email. You can use WMDecode tool to read this type of file.
Just configure the following :
Go to Contacts Folder in Outlook Express.
Double click on the recipient's e mail address.
In the Email property dialog box Select Internet Format. Change Plain ext only to HTML format.
Click on OK