Misconfigured Outlook clients used to be the bane of my existance. And, occasionally can still be, in 2012. Which is pretty astounding, when you think about it.
The background is pretty simple, really. Outlook clients send e-mail using a Microsoft proprietary format, which certainly isn’t MIME. That is pretty amazing that someone thought that everyone, everywhere, would use Outlook to send mail to other Outlook users.
Try explaining to a customer that those files aren’t a Notes/Domino problem Sure, they understand, but they want them to go away. So, it becomes my problem.
Thankfully, there are two notes.ini settings that you can apply to your SMTP server. Or server’s. Whatever the case may be.
Open the Domino Administrator. Go to the Servers tab, click on Server Console.
Enter “set config TNEFEnableConversion=1” then click either Send or hit enter.
Enter “set config TNEFKeepAttachments=1” then click either Send or hit enter.
To enable the setting to take effect immediately, enter “tell router update config” then click either Send or hit enter.
There, now someone else’s problem has been resolved on your servers. You’re welcome.
Here is a handy TechNote that explains the cause and resolution in a little more detail.
This behavior is due to messages being sent from Exchange without MIME encoding turned on. Winmail.dat is attached to the message in uuencoded format. Information on how to prevent winmail data being sent to Internet users from MS Exchange is documented and publicly available from the Microsoft Support Web page, in the Microsoft Knowledge Base, article number Q138053
Gregg Eldred 
Block them, send back something insulting the user 😀
Thanks for the tip Gregg. Been bugging me for ages. And I was tired of telling the ‘senders’ that they were in error.
TESTIFY!!!
I found a free program that unpacks the winmail.dat files. I usually have the user forward me the email, I unpack it, and send them back whatever was attached. This makes my life much simpler. Thanks!
Steve, I thought about mentioning something along that line but decided against it. Asking users (or Admins) to process those files just didn’t seem right to me. However, you are correct, there are several tools out there, including one from OpenNTF that handles them quite well.