You send an email with a file, but the recipient receives an unreadable file named winmail.dat. This problem occurs when Outlook uses a proprietary format that other email clients cannot process. This article explains why winmail.dat attachments appear and provides steps to ensure your files are sent in a universal format.
Key Takeaways: Fixing the Winmail.dat Attachment Problem
- File > Options > Mail > Message format > Internet Format: This setting forces Outlook to send all messages in a universal format, preventing winmail.dat files.
- Compose Window > Options tab > Format Text group > HTML or Plain Text: Sending a single message in HTML or Plain Text format avoids the proprietary RTF format that causes the issue.
- Control Panel > Programs > Programs and Features > Microsoft Office > Change > Quick Repair: Running a repair can fix corrupted settings that cause Outlook to default to the problematic TNEF format.
Why Outlook Creates Winmail.dat Attachments
The winmail.dat file is a container created by Microsoft’s Transport Neutral Encapsulation Format. Outlook uses TNEF to package advanced formatting, meeting requests, and voting buttons when it sends messages in Rich Text Format. Most non-Microsoft email clients like Gmail, Apple Mail, and Thunderbird cannot decode TNEF. They display the raw TNEF data as a single, useless attachment named winmail.dat or ATT00001.dat.
This issue primarily affects messages sent in RTF format to external recipients. Internal Exchange Server users typically do not see the problem because their Outlook clients understand TNEF. The problem is triggered by a default setting in Outlook that automatically uses RTF for messages to contacts in your address book. If a contact’s record is set to use RTF, all messages to that address will be sent with TNEF encoding.
Steps to Prevent Winmail.dat Attachments
You can configure Outlook to use universal formats for all messages or change the format for individual emails. The most reliable method is to change the global Internet Format setting.
Change the Default Message Format for All Emails
- Open Outlook Options
In Outlook, click File in the top-left corner, then select Options from the left-hand menu. - Navigate to Mail Settings
In the Outlook Options window, click the Mail category on the left sidebar. - Open Internet Format Settings
Scroll down to the ‘Message format’ section. Click the button labeled ‘Internet Format’. - Change the Encoding Option
In the Internet Format dialog, under ‘Outlook Rich Text options’, select ‘Convert to HTML format’ or ‘Convert to Plain Text format’ from the dropdown menu. Click OK to save. - Apply the Changes
Click OK again to close the Outlook Options window. All future messages to Internet recipients will now be sent in the chosen universal format.
Send a Single Email in a Universal Format
- Create a New Message
Click New Email to open a message composition window. - Select the Format Tab
In the new message ribbon, click the Options tab. Then, locate the Format group on the far left. - Choose HTML or Plain Text
Click either the HTML or Plain Text button. Avoid selecting the Rich Text button. The change applies only to this specific email. - Compose and Send
Add your recipient, subject, attachment, and body text. Click Send. The attachment will be sent in a standard, readable format.
If the Problem Persists or Recurs
Outlook Still Sends Winmail.dat After Changing Defaults
Check the individual contact setting in your address book. Open the People view, double-click the problematic contact, and click the Outlook Contact tab. In the Show group, click Details. Under ‘Internet format’, ensure the dropdown is set to ‘Let Outlook decide the best sending format’ or explicitly choose ‘Send using Plain Text’ or ‘Send using HTML’.
Recipients Report the Issue Only with Specific Attachments
Some advanced file types, like Outlook email templates with a .oft extension, are inherently tied to RTF format. For these, use an alternative method. Compress the file into a .zip archive before attaching it. Most email clients can extract .zip files, which bypasses the TNEF encoding problem entirely.
Winmail.dat Appears When Replying to a Thread
Outlook often matches the format of the original message in a thread. If you reply to an RTF message, your reply may also be sent in RTF. Before sending your reply, manually change the format using the Options tab method described above, even if your global defaults are set correctly.
Message Format Comparison: RTF vs. HTML vs. Plain Text
| Item | Rich Text Format (RTF) | HTML Format | Plain Text Format |
|---|---|---|---|
| Primary Use | Internal Exchange/Outlook communication | Standard web and email format | Universal compatibility, no formatting |
| Attachment Handling | Uses TNEF, creates winmail.dat for others | Sends standard MIME attachments | Sends standard MIME attachments |
| Formatting Support | Full Outlook formatting, colors, fonts | Basic fonts, colors, hyperlinks, images | No fonts, colors, or images |
| Recipient Compatibility | Microsoft Outlook and Exchange only | All modern email clients and webmail | All email clients without exception |
| Best For | Sending voting buttons or complex meeting requests internally | Sending formatted messages to any external contact | Maximum guarantee the recipient can read the content |
You can now prevent the winmail.dat problem by configuring Outlook’s Internet Format settings. For individual messages, remember to use the Options tab to select HTML before sending. A related advanced tip is to use the Microsoft Support and Recovery Assistant tool, which can automatically detect and fix TNEF-related send/receive issues in Outlook.