Fix Outlook Sending Winmail.dat Attachments

If you use Microsoft Outlook and recipients see a file named winmail.dat instead of a proper attachment, the fix is to change how Outlook formats your messages. This happens because Outlook uses the proprietary TNEF (Transport Neutral Encapsulation Format) for Rich Text messages, and the recipient’s email client cannot decode it. You can solve this by switching your message format to HTML or Plain Text, either for individual emails or globally for all outgoing messages.

Why Winmail.dat Files Appear

Outlook sends winmail.dat attachments when you compose a message using the Rich Text format. Rich Text is Outlook’s native formatting option, and it uses TNEF to encode formatting like fonts, colors, and special Outlook features (such as voting buttons or meeting requests). When the recipient uses a non-Microsoft email client—like Gmail, Yahoo, Apple Mail, or most webmail services—they cannot read TNEF. Instead of showing the formatted message, their client displays the raw TNEF data as a winmail.dat file. The actual attachments you intended to send are often buried inside that file, making them difficult or impossible to open.

Quick Fix: Change Format for a Single Email

If you only occasionally send attachments to people outside your organization, you can change the format for just that one message. This is the fastest solution and requires no permanent settings changes.

Open a new email message in Outlook.
Click the Format Text tab in the ribbon (in newer versions) or look for the Format group.
Select HTML instead of Rich Text. If HTML is already selected, switch to it explicitly—some accounts default to Rich Text without showing it.
Attach your files and send the message as usual.

HTML is the safest universal format. It preserves basic formatting like bold, italics, and colors, and it is readable by virtually every email client. Plain Text is even more universal but strips all formatting, so use it only when you are certain the recipient needs no styling.

Permanent Fix: Change Outlook’s Default Format

To stop winmail.dat files from appearing on all your outgoing messages, change the default format for new messages. This is the recommended fix if you regularly email people outside your company.

In Outlook, click File in the top-left corner.
Select Options (or Account Settings in some versions, then Options).
In the Mail category, look for the Compose messages section.
Find the drop-down menu labeled Compose messages in this format.
Change it from Rich Text to HTML.
Click OK to save the change.

This setting applies to all new messages you create. Existing drafts and replies to Rich Text messages may still use the original format, so check the Format Text tab when replying to such threads.

Fix for Outlook on Mac and Outlook on the Web

Outlook for Mac and Outlook on the Web (the browser version) do not use TNEF in the same way as the Windows desktop app. If you are using Outlook for Mac, go to Outlook > Preferences > Composing and ensure the message format is set to HTML. Outlook on the Web always sends HTML by default, so winmail.dat issues are rare there. If you still see the problem, it is usually because the sender used the Windows desktop client, not because of your own settings.

What to Do If You Receive a Winmail.dat File

If someone sends you a winmail.dat file, you can often recover the original attachments without asking the sender to resend. Several free online tools and standalone viewers can extract the embedded files. Search for winmail.dat viewer or TNEF opener to find a reputable option. These tools parse the TNEF data and list the original attachments, which you can then save. This is a temporary workaround—the permanent solution is for the sender to change their Outlook format settings as described above.

Common Misconceptions About Winmail.dat

A frequent misunderstanding is that winmail.dat is a virus or a sign of a corrupted email. It is neither. It is simply an encoding wrapper that Outlook uses for Rich Text messages. Another misconception is that changing the signature or disabling add-ins will fix the issue. While add-ins can sometimes interfere with attachment handling, they do not cause TNEF encoding. The only reliable cause is the message format setting. Also, note that Outlook’s Plain Text format never produces winmail.dat, but it also disables features like bold text, hyperlinks, and embedded images. If you need those features, HTML is the correct choice.

How to Check If Your Message Will Cause the Problem

Before sending, you can verify the format of the message you are about to send. Look at the title bar of the compose window—it may show the format name. More reliably, click the Format Text tab and see which option is highlighted. If Rich Text is highlighted, the recipient will likely receive a winmail.dat file if they use a non-Outlook client. Switch to HTML before sending. For recipients inside your own organization who also use Outlook, Rich Text is fine because Outlook can decode TNEF. The problem only appears when the recipient’s email client does not support TNEF.

Frequently Asked Questions

Why do I only see winmail.dat when sending to Gmail?

Gmail and other webmail services do not support TNEF. When Outlook sends a Rich Text message, Gmail cannot parse the TNEF data, so it displays the raw attachment as winmail.dat. Switching to HTML or Plain Text for messages to Gmail addresses eliminates the issue entirely.

Can I make Outlook automatically use HTML only for external recipients?

Outlook does not have a built-in rule that automatically switches formats based on the recipient’s domain. You can create a manual rule, but it is simpler to change the global default to HTML. HTML works fine for internal recipients too, so there is no downside to using it universally.

Will changing to HTML affect my email signatures or formatting?

No. HTML supports the same basic formatting as Rich Text for most purposes, including fonts, colors, images, and hyperlinks. Your signature should display correctly. The only features you lose are some Outlook-specific elements like voting buttons, which are rarely used in external emails anyway.

Is there a way to stop winmail.dat without changing my default format?

Yes, you can change the format per message as described in the Quick Fix section. However, this is easy to forget. If you regularly send to external recipients, changing the default is the more reliable long-term solution.

Does Outlook for Android or iPhone produce winmail.dat?

No. The Outlook mobile apps send messages in HTML format by default and do not use TNEF. The winmail.dat issue is specific to the Outlook desktop client for Windows when set to Rich Text.

What if the winmail.dat file contains an attachment I need urgently?

Use a TNEF viewer or an online extraction service to open the file. These tools will list the original attachments, which you can download. This is a one-time workaround; ask the sender to switch to HTML to prevent future occurrences.

General steps for current mainstream versions; menus can differ by device and version. See the disclaimer or report an error.

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