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We recently helped migrate one of our clients to Microsoft Hosted Exchange. Several weeks after the migration the Owner began experiencing a persistent problem sending file attachments. Every time he sent an attachment to certain contacts, all that would show up on the other end was a WINMAIL.DAT file. It didn’t matter what the original attachment file type was – we tried .pdf, .txt, .xls, .zip, etc. The root cause of this problem is TNEF, or Transport Neutral Encapsulation Formatting. If the message is sent using TNEF and the recipients’ email client does not have the capability of translating TNEF, the message can’t be ‘decoded’ and all that arrives is a binary WINMAIL.DAT file.

There are many posts online discussing this problem being related to Outlook and the Rich Text Format (which uses TNEF), such as this one: Even after following these steps and many others to make sure the Outlook 2007 mail client was not sending email in Rich Text Format, recipients were still getting winmail.dat in place of the attachments.

We discovered that somehow the Hosted Exchange Account had started transmitting this TNEF data no matter what format the Outlook mail client sent the message. If the mail format was HTML, RTF, or Plain Text the attachments would not be properly received. The problem was actually with the Server-side settings on his account, not Outlook as most of the Articles related to this problem will discuss.

Here’s how to fix it:

This is a link to the Microsoft Knowledgbase Article discussing TNEF and Hosted Exchange details:

To correct this problem for this customer on a Global level, we used PowerShell to manage the Hosted Exchange account, and executed 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.