What is a Winmail.dat Attachment

Do recipients of your emails, seemingly out of the blue, complain about a mysterious attachment called “winmail.dat” (or 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 mess? Does winmail.dat show up for some but not all recipients of your messages? 

It is not your fault. It is your Outlook's fault, in a way.

If Outlook sends a message using the RTF format (which is practically not used outside Outlook and Exchange) 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 will also commonly 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.

To prevent Outlook from attaching winmail.dat when you send an email:

1. Click File in Outlook.
2. Select Options.
3. Go to the Mail category.
4. Make sure HTML or Plain Text is selected for Compose messages in this format: under Compose messages.
5. Now make sure Convert to HTML format or Convert to Plain Text format is selected for When sending messages in Rich Text format to Internet recipients: under Message format.
6. Click OK.

Disable Winmail.dat Stubbornly Going to Particular Recipients No Matter the Default In Outlook 2016

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:

1. Make sure the email address is not in your Outlook Contacts. Outlook 2016 currently offers no way to change sending preferences for email addresses that are assigned to an address book entry.
2. Open an email from the desired email address or start a new message to it.
3. Right-click on the address.
4. Select Outlook Properties… from the menu that appears.
5. Make sure either Let Outlook decide the best sending format or Send Plain Text only is selected under Internet format.
6. Click OK.

Global Fix

To correct this problem for a customer on a Global level, use PowerShell to manage the Hosted Exchange account, and execute the following command:

• Set-RemoteDomain Default -TNEFEnabled $false

To confirm that the setting change was applied, use:

• Get-RemoteDomain |fl

In the returned results, look for:

• TNEFEnabled: False

The use of this command disables the transmission of TNEF information in ALL outbound messages to any/all remote domains.

For more information, see the original article.