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

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

To prevent this file from being attached to your email, please do 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:;en-us;q241538

Office 365

Set-RemoteDomain Default -TNEFEnabled $false

The above command was pulled from this website which disabled the dat file