|Is is an attachment that shows ATT next to it? If so then it is not meant to be read.|
Save the file to your desktop.
Try right clicking the file while holding down the Shift key and select Open With.
Select NotePad but do not set it to always use it.
That should read it.
Note: Mostly .dat files are irrelevant to anyone. They are almost always info about what is for a program to interpret and not the user.