What is the Winmail.dat file?

The Winmail.dat file holds information for the Rich Text message format. When you send a message to someone whose mail client doesn’t support Rich Text format (basically any other mail client which is not developed by Microsoft) or when it is improperly converted on the mail server, they will receive the message with the Winmail.dat file as an attachment.


Winmail.dat and missing other attachments

When a receiver whose client doesn’t support Rich Text format gets a message that has a Winmail.dat attachment, it could be the case that any other attachment that got sent with the message will be within the Winmail.dat file as well and therefore inaccessible for the receiver.

Prevent sending out a Winmail.dat attachment

To sending out Winmail.dat files, make sure you don’t send out Rich Text formatted messages to people who use a mail client which doesn’t not support it. Microsoft Office Outlook, Outlook Express, Windows Mail and Windows Live Mail are basically the only mail clients which support the Rich Text messages format.

The following shows the message format order from best to least compatible with other mail clients:

  • Plain Text
  • HTML
  • Rich Text

In other words; Only use Rich Text when you have an absolute need for it and know that the recipients mail client supports it.

Solution 1 – Changing the default message format

If you want to get rid of the Rich Text format default, you can change this in the following way;

  • Outlook 2007 and previous
    Tools-> Options-> tab Mail Format.
  • Outlook 2010
    File-> Options-> section Mail-> option: Compose messages in this format

You can still easily change back to Rich Text on a per message basis if you have a need for it. For this, see solution 3 below.

Changing the default message format in Outlook 2010.
Changing the default message format in Outlook 2010.

Solution 2 – Use Rich Text internally but not externally

If you prefer to use the Rich Text format for messages that you sent internally but want to send out in the HTML or Plain Text format for external contacts, you can set the following option:

  • Outlook 2007 and previous
    Tools->Options-> tab Mail Format-> button Internet Format…
  • Outlook 2010
    File-> Options-> Mail-> option group: Message Format-> option: When sending messages in Rich Text format to Internet recipients

Note:
Setting this option might not be needed for most Exchange environments as the Exchange server also holds this option and has its enabled by default. In that case, the conversion will take place at Exchange (server) level instead of at Outlook (client) level. In those cases it is also not possible to send out a Rich Text message to an external recipient even though the mail client of the external recipient supports it.

Solution 3 – Change on a per message basis

Regardless of your default preference, you can choose to change the message format to any other mail format when you have the need for it. Simply create a new message or use the Reply or Forward button and then follow the instructions below.

  • Outlook 2003 and previous with Outlook as the email editor
    Use the Format menu.
    Note: In order to change from HTML to Rich Text or vice versa, you’ll first have to select Plain Text before Rich Text or HTML becomes available.
  • Outlook 2003 and previous with Word as the email editor
    Use the drop down list on the E-mail toolbar.
  • Outlook 2007
    Use the Options tab on the Ribbon.
  • Outlook 2010
    Use the Format Text tab on the Ribbon

Solution 4 – Change on a per contact basis

You can also change the default message format on a per user or even a per e-mail address basis.

Note:
Checking the option mentioned below can also be the solution to preventing sending out a Winmail.dat file to a specific contact eventhough you are sending a message in the HTML or Plain Text format.

Outlook 2007 and previous

  1. Open the contact from your Contacts folder via a double click.
  2. Double-click on the e-mail address of the contact.
  3. In the Internet Format dropdown list choose the format you want to use for this specific contact’s e-mail address.

E-mail properties for Contact address
Changing the E-mail properties for the contact’s address.

Outlook 2010

  1. Open the contact from your Contacts folder via a double click.
  2. Hover on the email address of the contact and an Address Card pop-up will become visible.
  3. On the right side of the contact card you’ll see a menu icon button. Click on it to expand it and from the list choose; Outlook Properties
  4. In the Internet Format dropdown list choose the format you want to use for this specific contact’s e-mail address.

Outlook Properties for Contacts
The Outlook Properties option for the contact’s address is located on the new Contact Card. (click on image to enlarge)