Why do I get winmail.dat attachments?

The short answer

The sender’s Outlook sent the email in Outlook Rich Text Format. That format only works between Outlook and Exchange, so for everyone else Outlook wraps the formatting and the attachments in a package called winmail.dat (TNEF). If your email program isn’t Outlook, it can’t unwrap it. Nothing is wrong with your email account or device, and the sender usually has no idea it’s happening.

What the sender can change in Outlook

• Send new emails as HTML. In Outlook for Windows: File, Options, Mail, then set Compose messages in this format to HTML.
• Check how Outlook sends rich text to outside people. In the same Mail options, under Message format, set When sending messages in Rich Text format to Internet recipients to Convert to HTML format.
• Fix the contact. If it only happens to you, the sender’s saved contact for you may be set to always use rich text. Removing you from their autocomplete list (the suggestion that appears as they type your address) and typing your address fresh usually clears it.
• Replies keep the old format. Replying to a rich-text email stays in rich text, so a long thread can keep producing winmail.dat until someone starts a fresh email.

What the sender’s IT team can change

Companies using Exchange or Microsoft 365 can stop TNEF being sent outside the organisation altogether. In Exchange Online PowerShell, an administrator can run Set-RemoteDomain Default -TNEFEnabled $false, which makes Exchange convert outgoing rich-text email for external recipients instead of attaching winmail.dat. They can do the same for a specific domain if they’d rather not change the default.

A polite way to ask

“Hi, your emails are reaching me with a winmail.dat attachment instead of the files. Could you check that Outlook is set to send in HTML rather than Rich Text? If it’s a company setting, your IT team will know it as TNEF.”

Meanwhile: get the files out

You don’t need to wait for them. Open the winmail.dat on this page and the real attachments appear, ready to save, on a Mac, PC, iPhone, Android phone or Chromebook. It’s read in your browser and nothing is uploaded. More about opening winmail.dat.

Is winmail.dat dangerous?

The package itself is just a container. What’s inside deserves the same care as any attachment: if a file inside is a program, a script or a document with macros, the page points that out. Only open those if you trust the sender and were expecting them.