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"Do recipients of your emails, seemingly out of the blue, complain about a mysterious attachment called "winmail.dat" (of the even more mysterious content type "application/ms-tnef"), which they cannot open, no matter what they try? Do files you attach disappear in that winmail.dat moloch?
Does winmail.dat show up for some but not all recipients of your messages?
When, How and Why Winmail.dat-Application/MS-Tnef is Created.
It's Outlook's fault, in a way. Or the recipient's email client's. If Outlook sends a message using the RTF format (which is not very common outside Outlook) for bold text and other text enhancements, it includes the formatting commands in the winmail.dat file. Receiving email clients that do not understand the code therein display it as a stale attachment. To make matters worse, Outlook may also pack other, regular file attachments in the winmail.dat file.
Fortunately, you can get rid of winmail.dat altogether by making sure Outlook does not even try to send mail using RTF.
Prevent Winmail.dat Attachments from Being Sent in Outlook
To prevent Outlook from attaching winmail.dat when you send an email:
›› Step by Step Screenshot Walkthrough
Select Tools | Options... from the menu.
Go to the Mail Format tab.
Under Compose in this message format:, make sure either HTML or Plain Text is selected.
Click Internet Format.
Make sure either Convert to Plain Text format or Convert to HTML format is selected under When sending Outlook Rich Text messages to Internet recipients, use this format:
Click OK.
Click OK again.
Disabe Winmail.dat Stubbornly Going to Particular Recipients No Matter the Default
The standard settings for outgoing mail formats in Outlook can be overridden per email address. So, on a per case basis — when somebody complains about an inexplicable "Winmail.dat" attachment after you have made all the right settings changes —, you may have to reset the format for individual addresses:
›› Step by Step Screenshot Walkthrough
Search for the desired contact in your Outlook Contacts.
Double-click the contact's email address.
Alternatively, click on the desired email address with the right mouse button and select Outlook Properties... from the menu.
Make sure either Let Outlook decide the best sending format or Send Plain Text only is selected under Internet format:.
Click OK.
Extract Files from Winmail.dat without Outlook
If you receive winmail.dat attachments with embedded files, you can extract them using a winmail.dat decoder on Windows or Mac OS X.
Suggested Reading
Winmail Opener Review - Winmail.dat "
