How to fix issues with outlook attachments being replaced by winmail.dat

Configuring Outlook to Not Send Winmail.dat Attachments

We just setup a new tenancy on Office 365 and immediately hit this issue.

Sometimes when individuals on Office 365 powered by Exchange send messages containing pdf attachments, those attachments get converted to  a file called Winmail.dat that some other email clients can’t open properly.

Here is a bit of background to the issue:

What is Winmail.dat?

Outlook can use a method called Transport Neutral Encapsulation Format (TNEF) to package information for sending messages. The use of TNEF is affected by settings in Outlook that are referred to as Rich Text Format (RTF).

A TNEF-encoded message contains a plain text version of the message and a bundled binary attachment that contains the other parts of the original message. This attachment is named Winmail.dat, and includes:

  • The formatted text version of the message (including fonts or colors)
  • OLE objects (embedded pictures and embedded Office documents)
  • Special Outlook features (custom forms and meeting requests)
  • Any file attachments that were attached to the original message

Unfortunately, the Winmail.dat bundled attachment is not readable by many other email clients or by Outllook for Macintosh and it may even appear to the recipient as though the expected attachment is missing.

There are two ways to resolve this issue.  One is on the outlook client, the other is on the exchange server.

We’ll start with the outlook client:

How to Avoid Sending Winmail.dat Attachments

Setting Global Properties

Set the Global Properties to have Outlook by default send your email in Hypertext Markup Language (HTML):

  • Outlook 2007: Select Tools > Options> Email Format > Internet Options. Select Convert to HTML format.
  • Outlook 2010 and 2013: Select File >Options > Mail and then scroll to the bottom of the dialog. Select Convert to HTML format.

Meeting requests or voting buttons may not work with this setting, but you can override the format on select messages.


You can clear your cache settings for a person if they are still receiving Winmail.dat files from you. Create a new message, begin to enter the email address, then use the arrow keys to scroll up or down, highlight the address of the person and press the delete key to remove the entry. Enter their address in the To field and it will create a new entry.

You can also resolve this on the server via powershell with this command

Set-RemoteDomain Default -TNEFEnabled $false

This will tell exchange to consider all remote emails domains at not TNEF enabled.  You can also set them individually, but since it’s almost impossible to tell which domains will accept TNEF and which won’t it’s better to consider all to be unable to.