Winmail.dat files can prevent an email from being read correctly by either making the other attachment not available or removing some formatting within the body of the email.
The winmail.dat file type is a format that cannot be read by the AKO/DKO webmail. It is a Microsoft file type and can only be read by Microsoft products such as Outlook.
For the winmail.dat file type to appear, the message was sent using a Microsoft product most likely Outlook. Two of the most common reasons that an attachment of Winmail.dat may appear are the email being sent in a rich text format or the sender is attaching files using the “drag and drop” method.
This is not an exclusive list. There are many other reasons a winmail.dat file will appear on an email but to receive this file type within an email, the sender is using a Microsoft product.
Have the sender of the email check that their message is sent in plain text and that they are using the insert tool within Outlook or the other Microsoft product to attach the file to the email.
Even though the sender can be sending the email in plain text, their Exchange Server could be converting any email sent out from their server into another format such as rich text. The sender will need to contact their DOIM (Directors of Information Management) to find out if the message is being converted to another format.
If the sender cannot get the message to appear within AKO/DKO webmail without the winmail.dat attachment, either the sender should send the email within AKO/DKO webmail to prevent the issue from occurring or the recipient can configure their AKO/DKO mail to Outlook.
To learn more information on winmail.dat files, you can visit the Microsoft support page:http://support.microsoft.com/kb/278061