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 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