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CantOpen

Open winmail.dat

Someone sent you an email from Outlook and all you got is winmail.dat (or ATT00001.dat). The attachments they meant to send are inside. Get them out here, in your browser.

Drop your winmail.dat file here

or

Your file stays on this device: it is read here, in your browser, and never uploaded.

Also ATT00001.dat, or the whole email (.eml) that carries one. Up to 150 MB.

How it works

  1. Save the .dat file

    Save the winmail.dat attachment from your mail program. You can also choose the whole email saved as .eml.

  2. Drop it here

    We unpack it in your browser and list the files inside, with the message text if Outlook put it there.

  3. Save the attachments

    One by one, or all at once as a ZIP file.

winmail.dat, explained

What winmail.dat is

winmail.dat is a container that Microsoft Outlook and Exchange create in a format called TNEF (Transport Neutral Encapsulation Format). It carries Outlook-specific parts of a message: Rich Text formatting, voting buttons, meeting details, and the attachments themselves. Outlook unpacks it automatically. Other mail programs often do not, so the recipient sees a single file named winmail.dat or ATT00001.dat instead of the attachments.

Why you got one

The sender wrote the message in Rich Text format, or their Outlook or Exchange server decided to send Outlook’s own format to you. Microsoft’s documentation lists winmail.dat, ATT00001.dat and missing attachments as the typical symptoms.

How the sender can stop it

  • In Outlook: File → Options → Mail → “Compose messages in this format”: choose HTML or Plain Text instead of Rich Text. For a single message, choose HTML on the Format Text tab.
  • For one recipient: a saved contact or an autocomplete entry can keep forcing Rich Text; deleting that entry and typing the address again usually fixes it.
  • Exchange and Microsoft 365 administrators can switch TNEF off for outside recipients in the remote-domain settings (Set-RemoteDomain with TNEFEnabled set to false).

What this tool does and does not do

  • Lists and saves every attachment inside, with its full original name, or all of them as one ZIP file.
  • Shows the message text when the winmail.dat carries it (HTML, or Rich Text converted for reading).
  • Saves emails that were attached inside as .eml files.
  • Accepts the whole email (.eml) and finds the winmail.dat in it.
  • It cannot extract embedded OLE objects (for example a chart pasted from another program); those are listed but cannot be saved.
  • Meeting invitations are shown as text, not as a calendar entry.

Frequently asked questions

Is winmail.dat a virus?

No, it is a normal Outlook format. But it can contain any attachment, including a harmful one, so treat what is inside like any attachment from that sender.

What is ATT00001.dat?

Usually the same thing as winmail.dat under another name. Drop it here: if it is TNEF, its attachments appear.

Is my file uploaded?

No. It is unpacked in your browser and never leaves your device.

The tool says there are no attachments. Why?

Then the winmail.dat only carried formatting and text, and the real attachments, if any, travelled as separate files in the email. If the sender says they attached something, ask them to send it again in HTML format.

How do I open winmail.dat on an iPhone or Android phone?

Save the attachment to Files (iPhone) or Downloads (Android), then open this page in your phone’s browser and choose the file.

Can I stop receiving winmail.dat files?

Only the sender can. In Outlook they choose HTML or Plain Text instead of Rich Text (File → Options → Mail → “Compose messages in this format”), and an Exchange or Microsoft 365 administrator can turn this format off for outside recipients.

Sources