What is the winmail.dat attachment on some email messages?

Winmail.dat is a proprietary email attachment generated by Microsoft Outlook or Exchange Server when an email is sent using the Rich Text Format (RTF) or the Transport Neutral Encapsulation Format (TNEF) instead of standard MIME formatting.

What it contains

The winmail.dat file typically bundles information such as:

• The email’s RTF formatting (fonts, colors, tables)

• Embedded files or attachments

• Outlook-specific features (calendar invites, voting buttons, contact cards)

• Metadata used only by Outlook/Exchange

Why recipients see it

Recipients see a winmail.dat attachment when:

• The sender’s Outlook is configured to send RTF messages.

• The Exchange server forces TNEF encoding.

• The recipient’s email client (e.g., Apple Mail, Gmail, Thunderbird) does not understand the TNEF format and displays it as a generic file.

Is it harmful?

The file itself is not harmful, but it is not useful unless processed by Outlook or a TNEF viewer.

How to prevent it (sender side)

If you want to avoid sending winmail.dat files, the Outlook sender should:

1. Set Outlook to send messages in HTML or Plain Text:

   • File → Options → Mail → Compose in this message format → HTML.

2. Disable TNEF for external recipients:

   • File → Options → Mail → Message Format → “When sending messages to recipients using other email systems, use this format: Convert to HTML”.

3. Ensure that contacts in Outlook are not individually configured to use RTF.

How to open it (recipient side)

If you receive winmail.dat and need the actual contents, you can use tools such as:

• TNEF’s Enough (macOS)

• Winmail.dat Opener (Windows, iOS, Android)

• Online TNEF extractors