Hello,

Since a couple of weeks I keep getting calls about customers receiving winmail.dat files instead of a file.

After consulting with the IT manager, we narrowed down the area of impact and looked for a possible solution.

This is what we have found out:

1.My colleagues send attachments to multiple customers from an Exchange/Office 365 enviroment.
2.The customers are using different e-mail clients like: Windows Live, Apple Mail etc.
3.E-mails are being send to devices like an iPad and iMac (maybe more but I'd have to ask my IT manager).

What we have tried:

1.Changing from RTF to HTML in the e-mail options.
2.Running a PowerShell command that disabled the use of RTF in e-mails.

Both solutions did not work and we're clueless as to what the real culprit could be....

If there are any questions, then feel free to ask them!

Hi,

This usually happens when you are sending Rich Text format (RTF) emails via Microsoft Outlook to a recipient who is using a different email client.

Just like what you've tried, changing Outlook setting to convert the format of the message to the 'Plain text' or 'html' format should fix the issue. But after changing the settings, please dont forget to clear the auto-complete cache. The auto-complete cache (nickname cache) remembers the setting and Outlook will continue to send messages using RTF.

If issue persists, please try to create below registry key which will end TNEF encoding:

HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences

Add a new DWORD named "DisableTNEF" and set the value to 1.

Please have a try and feel free to post back with any findings.

Regards,

Ethan Hua
TechNet Community Support

Hi,

This usually happens when you are sending Rich Text format (RTF) emails via Microsoft Outlook to a recipient who is using a different email client.

Just like what you've tried, changing Outlook setting to convert the format of the message to the 'Plain text' or 'html' format should fix the issue. But after changing the settings, please dont forget to clear the auto-complete cache. The auto-complete cache (nickname cache) remembers the setting and Outlook will continue to send messages using RTF.

If issue persists, please try to create below registry key which will end TNEF encoding:

HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences

Add a new DWORD named "DisableTNEF" and set the value to 1.

Please have a try and feel free to post back with any findings.

Regards,

Ethan Hua
TechNet Community Support

Hi,

I'm glad the step works for you.

Regarding how to clear the auto-complete cache in the entire network with a script, I would suggest you post in our dedicated script forum for further assistance:

https://social.technet.microsoft.com/Forums/scriptcenter/en-US/home?forum=ITCG

I'm personally not familiar with this. Thanks for your understanding.

Regards,

Ethan Hua
TechNet Community Support

These articles may be helpful:

https://support.microsoft.com/en-au/kb/2487954

http://help.outlook.com/140/gg263346.aspx

http://robertkruk.me/2014/08/11/office365-and-winmail-dat-use-powershell-to-disable-tnef-message-formatting-with-remote-domains/

Thanks everyone for the links!

I'll post anything here if I run into any problems implementing the solutions from the links.