Usually people do not send a .dat file because it is a generic extension not associated with a particular program. Did they provide any indication of what kind of file it is supposed to be?

Please be very wary of mystery attachments, especially if you get instructions to rename them with dangerous executable extensions such as .exe, .bat, or .scr, since this could indicate someone trying to trick you into installing malware.

10:25 P.M. Thank you; your answer does help me a great deal. The attachment came with an email from TWC Support. The rep is trying to work it out and said: (1) Looks like it comes from Microsoft Outlook (which I do not use.) (2) TWC usually does not send attachments in their e mails.

Just got 2 more emails again with attachments. But, they were labelled delivery failure. I did not even bother to see who the sender was, just deleted them.

I am using AVAST Security for virus,malware and spyware. Am going to send them feed back. I do mean it about how helpful your answer was.  :)

Oh, is the attachment named "winmail.dat"? That is a file used by Outlook for "Rich Text Format" messages, which may be unreadable in other mail programs.