User ALWAYSMILEN3 posted the question below in the Ask Email Guy section, and I since this is a very common issue I decided to publish my answer up front here for everyone's benefit. This problem has been around for years now and still frustrates people who receive email from users of the various flavors of Microsoft Outlook.
I E-MAIL EXCEL FILES FROM WORK TO HOME AND WHEN I GET THEM ITS A .DAT FILE? WHY ISNT THIS LETTING ME GET MY EXCEL FILES THAT I HAVE FORWARDED? THX!
Posted by: ALWAYSMILEN3 | January 19, 2007 11:49 AM | (228)
My somewhat acerbic response (not at the user) is below:
Alwaysmilen3 - I'm going to break one of my published rules here and give you some help with your MS Outlook software, because this issue is a real pet peeve for me.
That winmail.dat file isn't created by Web Mail. It is a file that is created by Microsoft Outlook and Microsoft Exchange, and it is only understood by them. Learn more about it here.
In Microsoft's infinite wisdom they found it necessary to send attached files in a proprietary format rather than use MIME like every other email software in the world does. You can stop this from happening by changing the settings in those MS programs, so that when email is received by a non-Microsoft email reader (like our Web Mail) you won't get a useless winmail.dat file but instead you'll get the real attachment in its original format.
Here's Microsoft's explanation and fix for it: http://support.microsoft.com/kb/290809
You'll see them calling this a TNEF format like it is a real standard that other email programs are supposed to understand, but the fact is they entirely made it up and expected the world to support it (or force everyone to use Outlook). In fact, the full name of the format is MS-TNEF. The real standard in use by the rest of the universe is called MIME. Anyway, they tell you how to fix the problem there. Look down to the section that says "How to Specify E-mail Format".
There are also some freeware decoders you can download for those winmail.dat files. Find them on Google.
I generally like and use MS stuff, but winmail.dat has always irritated me in particular since I've never been an Outlook user. Since I felt the need to speak out on this topic, I probably should emphasize my disclaimer here (which appears elsewhere on this blog) that this comment is my opinion, and doesn't necessarily represent the position or opinion of EarthLink.
Ed. Note - Frequent commenter Bob corrected me in the comments below, pointing out that Outlook Express does not create these files, it is only Outlook and MS Exchange. Outlook Express can't read these messages either, and it doesn't even show you the winmail.dat, it just ignores it entirely. Correction has been made above to not include Outlook Express as one that creates these files.