Stop winmail.dat files.

Winmail.dat files are created when people send messages from incorrectly configured Outlook or Microsoft Exchange. These files are also known as TNEF (Transport Neutral Encapsulation Format).

This type of file normally contains formatting information. Winmail.dat files are not readable by most programs, so removes them automatically.

If a Subscriber attaches a valid file to a message, it is also converted into a Winmail.dat file. Therefore, Subscribers may be sending attachments, but no one on the Mailing List is receiving them.

Microsoft's explanation

Outlook, Outlook Online

Change the message format to HTML, Rich Text Format, or plain text
This info relates to these versions:
Outlook for Microsoft 365 Outlook 2019 Outlook 2016 Outlook 2013 Outlook 2010 Outlook 2007

Non-Microsoft Help for individual Outlook users

How to prevent Winmail.dat attachments from being sent in Outlook

Other issues to consider:

Address auto complete may cause the Winmail.dat file to be attached.

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