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PDF's sending as winmail.dat

Victoria 0 Reputation points
2025-11-18T16:23:52.8033333+00:00

sometimes when I send a pdf, the person receives it as a .dat file - this never use to happen - why is it happening now

Outlook | Windows | New Outlook for Windows | For home
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  1. AI answer

    2025-11-18T16:24:17.61+00:00

    When you send a PDF and the recipient receives it as a .dat file (often named Winmail.dat), it is typically due to the use of the Transport Neutral Encapsulation Format (TNEF) by Microsoft Outlook. TNEF is a proprietary format that can encapsulate rich text formatting and attachments, but not all email clients can interpret it correctly. If the recipient's email system does not support TNEF, they will receive the message as plain text along with the Winmail.dat attachment instead of the intended PDF file.

    This issue may not have occurred previously due to changes in email settings, updates in the email client, or variations in the recipient's email system. To prevent this from happening, you can configure your Outlook settings to send emails in a different format, such as HTML or plain text, which are more universally compatible with other email systems.

    You can change the message format in Outlook by going to File > Options > Mail and selecting either HTML or Plain Text under Compose messages in this format. Additionally, Microsoft 365 admins can use PowerShell to change settings that affect how messages are sent to external recipients, ensuring that Winmail.dat attachments are not included.

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  2. JANSHER LIWAG 8,850 Reputation points Independent Advisor
    2025-11-18T17:01:22.9466667+00:00

    Hi Kacper,

    Thank you for posting, I am here to help. I’m Sorry you are having this problem. Possible cause is the compatibility issue between Outlook and their email client.

    May i know if this is also happening to others aside from this receiver?.

    For the meantime try this steps let us check if this will help:

    Open Outlook application then click File and select Options. Choose Mail, in Compose messages section choose the format as HTML then click Ok or save. Reopen Outlook and try again.

    If still did not work try to use Outlook online OWA.

    open any web browser and copy paste this link to the browser. outlook.office365.com log in your Microsoft account. Try and check if send a pdf and still issue persist to the receiver.

    Let me know how this goes. Looking forward to helping you out.  If you need more help please feel free to let me know. If the answer is helpful, please click "Accept Answer".

    Best regards,

    Jansher

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