Recipients receive a winmail.dat attachment
Recipients notify you that the e-mail message you sent appears to them as a message with an attachment called winmail.dat.
You are using the Rich Text message format, which the recipient's e-mail program cannot interpret correctly.
Send your messages in either HTML or plain text format.
Send messages in HTML or plain text format
On the Tools menu, click Options, and then click Mail Format.
Under Message Format, in the Compose in this message format list, click HTML or Plain Text.