I started becoming aware of the winmail.dat problem a few years ago when smart phones really exploded. Users were complaining about winmail.dat email attachments on their phones but when they opened the email in Outlook it was a normal attachment. The issue was presented to me again because of a project in which we implemented a different email platform and some users were on Exchange and some users were on the new platform. Attachments were coming in as winmail.dat in the web client and on smart phones but not in Outlook.
So the root of this problem is as follows: In Exchange 2003 (probably Exchange 2000 also, I’m not sure), Microsoft had a default email format called “Exchange Rich Text Format”. Not to be confused with Rich Text (RFC 1341) or other standards based MIME formats, Exchange Rich Text Format is completely proprietary to Microsoft. So unless the email client (iOS, Android, Thunderbird, etc) supported this format the email recipient would receive a winmail.dat file for emails with certain types of attachments or email content (just got one in Gmail today!). Not being a recognized messaging standard, you can see why non-Microsoft clients might have a problem with it. Nonetheless, Microsoft has continued to support this format in to every subsequent version of Exchange and Outlook.
A good example of how to trigger this problem would be to insert a picture in to the body of an email from an Exchange 2003 mailbox and then send to somebody who wasn’t on Exchange.
Fixing the Winmail.dat problem
There are a fair number of articles out on the web that describe how to fix this problem from the Outlook client which I’m not going to repeat here. The solution I really wanted was how to fix this for all users on my Exchange server and prevent users from sending emails that the recipients couldn’t read. To accomplish this, turn off the Exchange Rich Text Format in Exchange.
In Exchange 2010:
|Organization Configuration -> Hub Transport -> Remote Domains (tab) -> right-click Default, select Properties||
|On the Message Format tab, under the Exchange rich-text format section, select “Never use”||
In Exchange 2003:
|Organization -> Global Settings -> righ-click Internet Message Formats, select Properties||
|On the Advanced tab, in the section Exchange Rich Text Format, change the radio button to “Never Use”||