How to stop winmail.dat attachments (and open the ones you get)

Why winmail.dat appears

Outlook can write messages as HTML, plain text or Rich Text. Microsoft explains that a Rich Text message is sent in a Microsoft format called TNEF (Transport Neutral Encapsulation Format). If the recipient’s mail system can’t process it, they get the plain text of the message and an attachment named winmail.dat, or sometimes a generic name such as ATT00001.DAT. The files the sender attached are packed inside it.

Outlook unpacks it automatically, so senders never see the problem. Apple Mail on a Mac or iPhone, and many other mail apps, show the package itself. Microsoft also notes that the file can include the sender’s sign-in name and the location of their Outlook data file.

If you received one: open it

1. Save the winmail.dat (or ATT…DAT) attachment to your device. On a phone, save it to Files or Downloads.
2. Open the winmail.dat opener and choose the file.
3. Save the attachments listed, one at a time or all as a .zip. The message text is shown as well.

If you saved the whole e-mail as an .eml file, you can choose that in the opener instead and click Open next to winmail.dat.

The file is read in your browser and never uploaded. Then ask the sender to change the setting below. You can send them a link to this page.

If you send them: change Outlook for Windows (classic)

1. Choose File → Options → Mail.
2. Under Compose messages, set Compose messages in this format to HTML.
3. Under Message format, check that When sending messages in Rich Text format to Internet recipients is set to Convert to HTML format, the default.
4. Click OK.

For one message, choose Format Text → HTML while writing it. Outlook replies in the format of the message you reply to, so check the format when replying to a Rich Text message. Some Outlook features still need TNEF: Microsoft lists meeting requests, voting buttons and task requests. Recipients outside Outlook may still get winmail.dat with those.

New Outlook for Windows and Outlook on the web

The new Outlook offers only HTML and plain text, in Settings → Mail → Compose and reply → Message format. Microsoft calls Rich Text a legacy format. If recipients still get winmail.dat, check the organisation’s mail server setting below.

For mail administrators (Microsoft 365 and Exchange)

Exchange decides whether Rich Text goes out as TNEF to other organisations. By default it follows each user’s Outlook setting. To turn it off for everyone:

• Exchange admin center: Mail flow → Remote domains, select Default (or the domain that has the problem), and set Use rich-text format to Never. Rich Text messages are then sent as HTML or plain text.
• PowerShell:

  Set-RemoteDomain -Identity Default -TNEFEnabled $false

• For one outside contact:

  Set-MailContact <address> -UseMapiRichTextFormat Never

Microsoft’s order is: the remote domain setting wins over the contact setting, which wins over Outlook’s own setting. The trade-off: Outlook users outside your organisation then lose what needs TNEF in your messages, such as voting buttons and custom forms.

If something looks wrong

• The winmail.dat holds no attachments. It sometimes carries only the message’s formatting. The opener then shows the text.
• The attachment is called ATT00001.DAT. It is the same kind of package. Choose it in the opener.
• It keeps happening after the sender changed Outlook. Their mail server sends Rich Text as TNEF (the administrator setting above), or the message was a meeting request or had voting buttons.