How to stop winmail.dat files

Winmail.dat or TNEF (Transport Neutral Encapsulation Format) files are created by Outlook or Microsoft Exchange.  These files are not readable by normal programs, and mail-list® removes them automatically.

So when users have included a valid attachment, by the time it's put onto the Internet, it has been changed to a Winmail.dat file.

Therefore, people are sending attachments, but nobody on the mailing list is receiving them.

To adjust an individual's Outlook settings, see

On Office 365, adjust the server settings to resolve this problem for all users

Address auto completion may cause the winmail.dat file  

Experiment with typing the full email address, instead of selecting an address after typing a few letters.