Sometimes when sending email from a Microsoft email app such as Outlook or Microsoft Exchange, there might be a winmail.dat file attached to the message. The attachment doesn't appear in the Microsoft email app, but it does appear in iPhone Apple Mail, and in other apps it may appear as a MIME section named "application/ms-tnef."

Winmail.dat files contain the formatting of the email such as font, color, boldfacing, or underlining, and as it is a Microsoft proprietary standard format, the file is sometimes not recognised by other email clients.

If you use Outlook or Microsoft Exchange, to prevent this file from being attached to your email, please use the following simple steps to send your messages in either HTML or plain text format:

1. On the Tools menu in Outlook, click Options, and then click Mail Format.
2. Under Message Format, in the "Compose in this message format” list, click HTML or
   Plain Text.
3. Then click OK

For more information see: https://support.microsoft.com/en-us/help/290809/how-e-mail-message-formats-affect-internet-e-mail-messages-in-outlook