Email Attachments are Saved as Winmail.dat instead of the Original File Format

Any email sent from an Exchange Server to a non-Exchange mail reader contains an attachment called winmail.dat. Using an Exchange Server to send the email, user won't see this file, and the message will retain its formatting. However, it can be confusing for those who don't use an Exchange Server.

For reference, review this article regarding messages being sent out from Exchange and there being a setting in Exchange server which converts the word doc to winmail.dat file. This needs to be setup from the sender's end, the attachment for the case is being sent by your customer.

Below are some ways to control whether or not to send messages in RTF, follow the option that best meets the situation:

Note: When an end user sends mail to the Internet from an Exchange Windows or Outlook client, a file attachment called Winmail.dat may be automatically added to the end of the message if the recipient's client cannot receive messages in Rich Text Format (RTF). The Winmail.dat file contains Exchange Server RTF information for the message, and may appear to the recipient as a binary file. It is not useful to non-Exchange Server recipients.

• Creating Custom Recipients.When an administrator creates a custom recipient using the Microsoft Exchange Administrator program, click to clear the Always Send To This Recipient In Microsoft Exchange Rich-Text Format check box.
• Modifying Existing Microsoft Exchange and Custom Recipients.An administrator can prevent an existing user account (Microsoft Exchange user or custom recipient) from sending RTF information by clicking to clear the MAPI Recipient check box on the Advanced property page of the recipient's properties. An administrator can view the recipient's properties by clicking the recipient name, and then clicking Properties on the File menu.
• Addresses in the Personal Address Book.End users can modify the Internet addresses in a personal address book (PAB) to prevent sending RTF information by clicking to clear the Always Send To This Recipient In Microsoft Exchange Rich-Text Format check box in the SMTP - Address property page of the Internet address in the PAB. To obtain the properties of an entry in the PAB, click the entry, and then on the File menu, click Properties.

Configuring the Internet Mail Connector (IMC).An administrator can configure the IMC with RTF options in the following manner: 

          1. Open the Internet Mail Connector Properties page.
          2. Click the General tab.

 The Send Microsoft Exchange Rich Text list box controls the sending of rich-text data. There are three values to choose from:

• If the value is set to User, the recipient properties are used to determine whether or not to send RTF information.
• If the value is set to Always, RTF information is always sent, regardless of the recipient properties.
• If the value is set to Never, RTF information is never sent.

          An administrator can also configure the option to send RTF information on a domain-by-domain basis. To define e-mail domains and the message settings for that domain, click the E-Mail Domain button of the Internet Mail tab.
     
Note: The above information are found and is in reference to Microsoft's KB Article.