For Google Workspace.

For Microsoft 365.

Question:

Using Outlook as mail client then a recipient says that he/she recieved a mail attached "Winmail.dat" file.

What is "Winmail.dat" file?

And is this attached file encrypted?

Answer:

In the mail client which is provided by Microsoft Corporation, when the mail take partial format (Rich Text Format), it might send to the mail which is included Winmail.dat attachment. 

・Microsoft support page

https://support.microsoft.com/en-us/kb/278061 

If the receiving client is not Microsoft, the mail which includes a Winmail.dat attachment might recognize as  attachment file.

Our HENNGE one also recognize Winmail.dat as attachment file, so ZIP encryption function is applied Winmail.dat encryption.

However, in filtering service setting you set Winmail.dat file as "File Extensions Skipped Encryption".

For Microsoft 365, Winmail.dat file is registered as "File Extensions Skipped Encryption" by default.