When an Outlook user composes and sends a message using Rich Text Format, Outlook automatically generates a file called winmail.dat and attaches it to the message. The winmail.dat file contains formatting information (in an unreadable format) that Outlook will use on the receiving end to display this email message correctly. Unfortunately, Outlook is the ONLY mail client program that can use this format and extract any information from these files. Thunderbird, Eudora, and other mail client programs don't understand this format or the information contained in these files.

To resolve this issue you can change your default sending mode in Outlook to Plain Text. To change the default sending mode:

In Outlook 2010/2013:

  1. Open the File tab on the Ribbon.
  2. Choose Options.
  3. In the Outlook Options window, choose Mail and scroll down to the Message Format section.
  4. In the last dropdown, select Convert to Plain Text format.
  5. Click OK to save changes.

In Outlook 2003/2007:

  1. Select Tools > Options from the Outlook menu bar.
  2. Click the Mail Format tab.
  3. Under Message Format, select Plain Text.
  4. Click OK to save changes.

Now your recipients should receive your attachments in their original format instead of seeing the winmail.dat file.

For more information on how to set the specific contact format or the specific message format in Microsoft Office Outlook 2003 or 2007, please refer to the following Microsoft Knowledge Base article: E-mail attachments are not visible to some recipients (http://support.microsoft.com/kb/2534447/en-us).