Hi Neal,
Thanks for contacting us and sorry for the inconveniences,
Cited from the article https://www.thewindowsclub.com/stop-sending-or-receiving-winmail-dat-attachments-outlook To stop Outlook from sending winmail.dat attachments. You can try the following methods and see which one works for you.
"1] Change the Compose messages setting in Outlook Open Microsoft Outlook. Go to “File > Options.” This will open the Outlook Options window. Select the Mail category from the left side. Click on the Compose messages in this format drop-down and select HTML or Plain Text. You will find this option under the Compose Messages section. Scroll down and locate the Message format section. Select Convert to HTML format or Convert to Plain Text format in the “When sending messages in Rich Text format to Internet recipients” drop-down. Click OK to save the settings.
2] Type the email address of the recipient manually Many users have found this solution helpful. If you select the email address of your recipients from the Address Book, try this trick. Type the email address of the recipient manually instead of selecting it from the Address Book and see if it helps."
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Let me know if this helps or if you need further assistance.
Regards, Sola
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