Receiving Winmail.dat files on my iPhone

I was receiving , among others, Winmail.dat files instead of a PDF of JPG file that was sent from my Outlook mail server. But not all recipients of an email would get the Winmail attachment, some got the proper file ??? weird. I checked all outlook settings and changed with nauseum. I found the answer berried on the internet.

One ; I did not change anything (same result from another PC that I’ve histotically signed into). 

Under Outlook Email Options, Composed messages were HTML, and In Message Format, Convert to HTML Format was selected

Two; When checking email formats , The sent Emails showed "Message - (HTML)" with the in the email header when clicked on. 

Three: The winning solution! I tested successfully with four separate users with success. I did not have these users (or any others) in my contacts. They were all cached. After I deleted their cached email accounts from the "To field" under "Recent People" when hoovering over address bar I then sent them the PDF again, each one responded that they could open it successfully.