For several years now I have faced the "winmail.dat" attachment issue, which is caused by an incorrectly configured Outlook email client. During this issue, every email that is send from an MS Outlook client, to a recipient that uses an email client different than Outlook (e.g. Thunderbird), it contains an attachment called "winmail.dat", that cannot be properly recognized (and opened) from the recipient's email client.
The "winmail.dat" issue occurs in all Outlook versions (2003, 2007, 2010 or 2013), because the email sender is using the "Rich Text" format to compose email messages.
Microsoft is aware of the "winmail.dat" attachment issue and as a resolution of this problem, proposes to configure the Outlook client to compose messages either in HTML or Plain Text format. But, in several cases, despite of this setting applied, the recipient continues to receive e-mails containing the "winmail.dat" attachment.
This tutorial contains detailed instruction on how to prevent Outlook (2003, 2007, 2010 or 2013), from sending the "winmail.dat" file to recipients.
How to STOP Outlook from sending the Winmail.dat attachment file.
Solution-1. Send Email in HTML or Plain Text format.
Configure the message composing in Microsoft Outlook client (at sender's side) in Plain Text or HTML format, as Microsoft suggests. *
1. If you are using Outlook 2007, first download and run the hotfix 957692 from Microsoft (KB957692), before you apply this solution.
2. Keep in mind that this solution doesn't resolve the "winmail.dat" problem, in all cases. If, after you have configured the "compose message format" setting to "HTML" or "Plain Text", your recipients are still receiving your messages with the "winmail.dat" attachment, then proceed to apply the solution-2 below.
Outlook 2003 & Outlook 2007
1. Go to Tools menu and click Options.
2. At Mail Format tab, set the Compose in this message format: to Plain Text or to HTML. (From my experience the Plain Text setting, is the only setting that prevents Outlook to send the Winmail.dat file.)
3. Then choose the Internet Format… button, and under "Outlook Rich Text" options, click either:
a. Convert to Plain Text format, if you have set the Compose in this message format to Plain Text at previous screen, or
b. Convert to HTML format, if you have set the Compose in this message format to HTML at previous screen.
4. Hit OK twice to exit.
5. Close Outlook.
6. Proceed to delete Outlook's Autocomplete cache as described at the end of this article.
7. Open Outlook. If the "WINMAIL.DAT" problem persists continue to solution-2 below.
Outlook 2010 & Outlook 2013
1. From the File menu, click Options.
2. Choose Mail on the left pane and then set the Compose in this message format: to HTML or to Plain Text *. Press OK when done.
* Note: From my experience the Plain Text setting is the only setting that prevents Outlook from sending the Winmail.dat file.
3. Close Outlook.
4. Proceed to empty Outlook's Autocomplete cache as described at the end of this article.
5. Open Outlook. If the "WINMAIL.DAT" problem persists continue to solution-2 below.
Solution 2. Registry FIX to prevent Winmail.dat (Outlook 2007, 2010 or 2013)
Step 1. Open Registry Editor & Backup Registry.
1. Close Outlook.
2. Open Registry editor. To do that:
- Simultaneously press the “Win” + “R” keys to load the Run dialog box.
- Type regedit and press Enter to open Registry Editor.
IMPORTANT: Before you continue, first take a backup of your registry. To do that:
1. From the main menu, go to File & select Export.
2. Specify a destination location (e.g. your Desktop), give a filename to the exported registry file (e.g. “RegistryUntouched”), select Export range: All and press Save.
Step 2. Modify TNEF setting in Windows Registry.
1. After you have backed up your registry, navigate (from the left pane) to the following key, according to the Outlook version you installed.
2. Right-click on an empty space at the right pane and select New > DWORD Value.
3. Give the name DisableTNEF to the new value and press Enter.
4. Double click at DisableTNEF and at the Value Data type 1 and press OK.
5. Close Registry Editor.
6. Open Outlook. The "winmail.dat" problem should be gone! *
* Additional help: If the "winmail.dat" attachment problem persists, close Outlook and then delete (clean) the Outlook autocomplete cache, by following the instructions below.
How to empty the Autocomplete cache in Outlook.
In Outlook 2003 & Outlook 2007:
2. Type the following and press Enter:
3. Delete the Outlook.NK2 file.
In Outlook 2010 & Outlook 2013.
2. Type the following command and press Enter:
- outlook.exe /cleanautocompletecache
You 're done! Let me know if this guide has helped you by leaving your comment about your experience. Please like and share this guide to help others.