This problem is often attributed to TNEF (a type of mail format)
If Microsoft Outlook Rich Text Format is selected, then the recipient's
server may have trouble decoding the format, and will thus convert the
mail to DAT format.
The solution is to change the settings in your Email Client Program so
that mail is sent in either HTML or Plain Text format (not Rich Text).
For example, in Outlook, go to Tools menu, click Options, and then click
the Mail Format tab. In the "Send in this message format" list, select
Plain Text or HTML, and then click OK. "HTML format" will retain all "MS
Word" document formatting while "Plain text" is just that...similar to
what you see in Notepad.