Description1) Inbound emails received from an external sender includes a winmail.dat attachment.
2) Outbound emails from within your organization includes a winmail.dat attachment.
Solution1) If you are receiving emails from external senders with a winmail.dat attachment, it is most likely that the sender is sending the email in Rich Text Format using Microsoft Outlook.
Source: https://support.microsoft.com/en-gb/kb/278061
The winmail.dat file is used to preserve Rich Text formatting. Microsoft Outlook uses it when sending a Rich Text-formatted message. During transport, the content of the message may be changed, preventing the receiving client from being able to read the formatting instructions. In other cases, the receiving client does not use or recognize the winmail.dat file.
If the email is received from an external sender you can ask the sender to re-send the message in plain text format.
2) 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.
Source: https://support.microsoft.com/en-gb/kb/138053
