Outlook — Recipients Receive Winmail.dat Attachments
When sending an attachment to a recipient using Outlook, the attachment arrives as a file called WINMAIL.DAT. Sometimes the attachments can't be opened.
This may occur if the contact record in the sender's address book is corrupt, or the setting to use Rich Text Format for that contact is enabled.
As the latter setting is not easily changed in modern versions of Outlook, a simple solution is to delete the contact record and recreate it. Also delete the cached Nickname for that contact.
To access this dialog, open a command prompt and enter the following commands:
reg add HKCU\Software\Microsoft\Office\16.0\Common\ContactCard /v TurnOnLegacyGalDialog /d 1 /t REG_DWORD /f
reg add HKCU\Software\Microsoft\Office\13.0\Common\ContactCard /v TurnOnLegacyGalDialog /d 1 /t REG_DWORD /f
Then search for the contact using F11 and double-click the result. This will enable setting the Internet Format.