Objective:

• Winmail.dat file forwarded in with Email
• Winmail.dat file attachment forwarded with Email to Maildrop

Environment:

• Clio Manage

Additional Information:

• Work around if using MailDrop: Be sure to copy and paste the Matter Maildrop address every time an email is forwarded. Using the saved address within Outlook appears to cause the file to change into a Winmail.dat format. 
• If you would like to turn off TNEF for Outlook 2010 and later versions (i.e. to stop the winmail.dat attachment from appearing in emails), click here.

Answer:

Please check out this Microsoft Support article for more information: 

• Email received from a sender using Outlook includes a Winmail.dat attachment