Why are some of my email recipients receiving my attachments that read 'winmail.dat

Question

When sending emails with attachments, some recipients are unable to open the attachment. I had a recipient send me a screenshot and it says winmail.dat

Can I make changes on my end so that all recipients can receive attachments from me or does the problem lie on their end?

Thanks,

Answer 1

When I went in to change to HTML or Plain Text, it was designated as HTML. I changed it to Plain Text. I have not been able to visit with those affected to see if the change helped. As soon as I do, I will let you know.

Thanks,

Answer 2

Hi @Dusty Lynch,

Welcome to Microsoft Q&A forum.  

Thank you for reaching out about the issue where some recipients are receiving a winmail.dat file instead of your intended attachments.

Some recipients see a winmail.dat file because Outlook is sending those messages using Rich Text Format (RTF) with TNEF (Transport Neutral Encapsulation Format). Many non-Microsoft email apps can’t interpret TNEF, so instead of your actual attachment, they display an unreadable winmail.dat file.

You can fix this on your end so all recipients receive standard attachments. Here are my recommendations:

1. Use HTML or Plain Text instead of Rich Text

In New Outlook for Windows:

• Go to Settings (gear) → Mail → Compose and reply
• Under Message format, select HTML (or Plain Text)
• Save changes and resend a test email with an attachment.

2. Clear per-contact Rich Text settings

Outlook may remember to use RTF for certain addresses via autocomplete. To reset:

• Start a new email → type the recipient → when the autocomplete suggestion appears, click X to remove it → re-add the recipient and send again.

3. If your organization uses Microsoft 365 or Exchange Online

Contact your IT admin to disable TNEF for external domains:

• In Exchange Admin Center → Mail flow → Remote domains → Default → Edit text and character set, set Use rich-text format to Never.

I hope this information is helpful. Please follow these steps and let me know if it works for you. If not, we can work together to resolve this.    

Thank you for your patience and your understanding. If you have any questions or need further assistance, please feel free to share them in the comments on this post so I can continue to support you. 

I look forward to your thoughts on this. 

Wishing you and your family a joyful and prosperous New Year. 

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Answer 3

The issue you're experiencing with recipients receiving attachments labeled as 'winmail.dat' is related to the TNEF (Transport Neutral Encapsulation Format) used by Outlook. When you send emails with certain formatting or features, Outlook may encode the message in TNEF, which is not universally supported by all email clients. As a result, recipients using email clients that do not understand TNEF will receive the plain text version of your message along with a 'winmail.dat' attachment, which they cannot open or use effectively.

To ensure that all recipients can receive your attachments without issues, you can change your email format settings in Outlook. Here are the steps you can take:

1. Disable TNEF globally: You can modify the Windows registry to disable TNEF for all outgoing messages. This will prevent the 'winmail.dat' file from being sent.
2. Change per-message settings: When composing an email, you can select the message format as either HTML or Plain Text instead of Rich Text, which uses TNEF.

By making these changes, you can avoid sending 'winmail.dat' attachments and ensure that your recipients can access your attachments without any issues.

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References:

• How message format affects email messages
• Message format and transmission in Exchange Online