The file of “winmail.dat” allows Outlook users to send Transport Neutral Encapsulation Format (TNEF) information along with the email which retains the look and feel of the document with formatting, fonts, and colors when its received by another Microsoft Outlook user, but is not useful to non-Exchange Server recipients.
The Transport Neutral Encapsulation (TNEF) Format allows Outlook users to send information in an email along with informations which retains the look and feel of the document with formatting, fonts, and colors across an Exchange environment. However it is not useful to non-Exchange Server recipients and if not converted (see here) on that system, it will show as winmail.dat file/attachment.