Had a customer reporting that their clients receive WinMail.dat file as an attachment instead of the email that they intended. According to Microsoft Support:
The Winmail.dat file is used to preserve Rich Text formatting. Outlook uses it when sending a Rich Text-formatted message. During transport, the content of the message may be changed, preventing the receiving client from being able to read the formatting instructions. In other cases, the receiving client does not use or recognize the winmail.dat file.
Two ways to stop clients receive WinMail.dat file as attachment
Either send everyone plain text emails or change the settings on a per contact basis inside of Outlook. Here is a link for how to do it on a per contact basis, this can be tricky but worth it if you want to keep the rich text formatting of email for the rest of your contacts.
If your company is using Exchange or Office 365 for email and need help troubleshooting, then contact us for assistance.