Email, Messaging, & Video Calls Email Learn the Correct Way to Prevent Auto-Sent Winmail.dat Attachments Addressing this known issue in Outlook Share Pin Email Print PM Images / Getty Images Email Yahoo! Mail Gmail By Heinz Tschabitscher Writer A former freelance contributor who has reviewed hundreds of email programs and services since 1997. our editorial process Heinz Tschabitscher Updated February 03, 2020 When you send an email from Outlook, an attachment called Winmail.dat is sometimes appended to the end of your message, whether or not your recipient has elected to receive emails in Rich Text Format or in plain text. Normally, the attachment appears in binary code, which isn't useful. Microsoft acknowledges this is a known issue in Outlook 2016 for Windows and earlier versions of Outlook. It sometimes occurs even when everything is set to use HTML or plain text. The known issue hasn't been resolved. However, Microsoft recommends a few steps that may minimize the problem. 01 of 03 Recommended Settings for Outlook 2016, 2013, and 2010 In Outlook 2016, 2013, and 2010: Select Tools > Options > Mail from the menu and scroll to the bottom of the dialog screen.Next to When sending messages in Rich Text Format to internet recipients: select Convert to HTML from the menu.Click OK to save the setting. 02 of 03 Recommended Settings for Outlook 2007 and Earlier In Outlook 2007 and older versions: Click Tools > Options > Email Format > Internet Options.Select Convert to HTML Format in the Internet Format dialog window.Click OK to save the setting. 03 of 03 Set the Email Properties for a Contact If a specific email recipient is receiving the Winmail.dat attachments, check the email properties for that specific recipient. Open the Contact.Double-click on the email address.In the Email Properties window that opens, select Let Outlook decide the best sending format.Click OK to save the setting. Let Outlook Decide is the recommended setting for most contacts.