What Winmail.dat Is and Why You Can't Open It Normally

A winmail.dat file is a container that Outlook creates when you send a formatted email — one with special fonts, colors, tables, or attachments — to someone using a non-Outlook email program. Instead of letting the recipient see your formatting, Outlook wraps it all into this single file and attaches it to your message. The recipient gets an email with an attachment they cannot read, because winmail.dat is a proprietary Microsoft format.

You cannot open winmail.dat by double-clicking it. Your computer does not have a default program for it, and most email clients do not recognize the format. The file contains the original message and its attachments, but they are encoded in a way that only specific tools can decode.

The real solution depends on whether you sent the file (and can resend it differently) or received it (and need to extract what is inside).

Key Takeaways

  • Winmail.dat files are created by Outlook when sending formatted emails to non-Outlook users, and they cannot be opened with a standard double-click.
  • If you sent the file, the fastest fix is to resend the email as plain text from Outlook, which prevents the winmail.dat wrapper from being created.
  • If you received a winmail.dat file, you can extract its contents using free online converters, specialized desktop software, or by asking the sender to resend in plain text.
  • Online converters like Winmail Viewer or CloudConvert let you upload the file and read the extracted attachments without installing software.
  • Outlook users can open winmail.dat by dragging it into a new email message, which forces Outlook to decode and display the contents.

Resend the Email as Plain Text (If You Sent It)

If you are the one who sent the winmail.dat file, the simplest fix is to send the email again, but this time formatted as plain text instead of HTML or rich text. This tells Outlook not to create the winmail.dat wrapper.

Open Outlook and create a new email to the same recipient. Type your message and attach any files you need to send. Before you click Send, look for the Format menu at the top of the window. Click Format, then select Plain Text. Now send the email. The recipient will receive your message and attachments without any winmail.dat file attached.

If you are replying to an email instead of starting a new one, the same steps explore — just switch to Plain Text format before sending.

Use an Online Converter to Extract the Contents

If you received a winmail.dat file and need to see what is inside, an online converter is the fastest route because you do not need to install anything. Several free services will decode the file and let you read the original attachments and message content.

Go to a converter website like Winmail Viewer (winmailviewer.com) or CloudConvert (cloudconvert.com). On Winmail Viewer, click the upload button, select your winmail.dat file from your computer, and click Convert. The site will show you the message content and list any attachments. Click the attachment name to read it to your computer.

CloudConvert works similarly — upload the file, select the output format you want (usually you will choose to extract all files), and click Start Conversion. read the results when they are ready. These services are free for single files, though some limit file size or number of conversions per day.

Open It in Outlook If You Have the Program

If you use Outlook yourself, you can force it to decode a winmail.dat file by dragging the file into a new email message. Create a new email in Outlook, then drag the winmail.dat file from your file explorer and drop it into the message body. Outlook will recognize the file and decode it, showing you the original message and attachments.

This method works because Outlook understands its own format. Once you see the contents, you can copy the message text or save the attachments to your computer. This is faster than using an online converter if you already have Outlook open.

Install Specialized Software for Repeated Use

If you receive winmail.dat files regularly, installing a dedicated tool may save time. Programs like Winmail Decoder (for Windows and Mac) or eM Client (which has built-in winmail.dat support) let you open these files directly without uploading to a website.

Winmail Decoder is a small program that adds a right-click option to winmail.dat files — select Open With Winmail Decoder and it extracts the contents when ready. eM Client is a full email program that handles winmail.dat files automatically, so if you switch email clients, it will open them without any extra steps.

These tools cost money (Winmail Decoder is around $20, eM Client has a free version with limitations), so they make sense only if you deal with these files often. For occasional use, the online converters are free and just as fast.

Ask the Sender to Resend Without Formatting

If you cannot open the file yourself and the sender is willing to help, ask them to resend the email as plain text. This is the most reliable fix because it prevents the winmail.dat file from being created in the first place.

Tell the sender: "I received your email with a winmail.dat attachment, which I cannot open. Could you resend it as plain text instead of formatted text?" If they use Outlook, they can follow the steps in the "Resend the Email as Plain Text" section above. If they use a different email program, they usually just need to turn off rich text or HTML formatting before sending.

Frequently Asked Questions

Is winmail.dat a virus or malware?

No. Winmail.dat is a legitimate file format created by Outlook. It is safe to open and extract. However, like any email attachment, you should only open files from senders you trust, because the attachments inside could contain malware.

Why does Outlook create winmail.dat when I send to Gmail or Yahoo?

Outlook creates winmail.dat when you send formatted emails (with special fonts, colors, or tables) to email programs that do not understand Outlook's formatting. Gmail and Yahoo can read basic HTML, but Outlook sometimes wraps the message anyway to preserve exact formatting. Sending as plain text prevents this.

Can I delete the winmail.dat file safely?

You can delete it if you have already extracted the attachments you need. However, if you delete it without opening it first, you will lose access to any attachments or message content inside. Always extract the contents before deleting.

Do I need to change my Outlook settings to stop sending winmail.dat files?

You do not need to change settings permanently. Just switch to Plain Text format before sending each email to a non-Outlook user. If you want to change the default, go to File > Options > Mail > Compose Messages, and select Plain Text as your default format.

What if the online converter does not work on my file?

Try a different converter — some handle corrupted or unusual winmail.dat files better than others. If multiple converters fail, the file may be damaged. Ask the sender to resend the email instead.