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Winmail.dat receiving in outlook 2013

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A user in our company is receiving winmail.dat attachments, she is migrated to office 365 and is using office 2013, the same email received by a user  that isn't migrated and still using office 2010 is receiving the same email with the correct attachment. The received mail is formatted in Plain text.

Any ideas on how to resolve this?


All Replies
  • Hi William,

    To clarify the issue, I'd like to collect the following information first:

    1. Do you have Hybrid environment deployed?

    2. Whether the user who has been migrated to Office 365 has this issue or the user who hasn't been migrated to Office 365 has this issue.

    3. From the description, I see "A user in our company is receiving winmail.dat attachments". I'd like to confirm whether the issue persists if the attachments are other file types, such as Excel, Word and so on.

    I'd appreciate it if you could post the information here.

    Aaron Yuan

  • 1. We have a hybrid environment.

    2. It's the user that has migrated already that has the problem.

    3. we have seen the problem with different file types such as excel and pdf for example.

  • Hi William,

    Could you help us collect the following information to narrow down this issue?
    1. If you login to Office 365 OWA, do you receive the correct attachment?
    2. Please send emails by using plain text mode, will the user receive winmail.dat attachments?
    3. If the user of Outlook 2010 sends an email to the Office 365 user, can she receive the correct attachment?

    Edward Qu

  • There is a similar thread for your reference:

    0 out of 1 people found this post helpful.

  • Hi Edward,

    1. When login to the OWA we still see an winmail.dat file

    2. Reveived mail is in plain text format

    3. When the user of outlook 2010 sends to the office 365 user, the attachment is correct.


  • Hi Chris_123,

    Thanks for looking into my problem, but if you read carefully you can see that these two problems are completly differetn.

    I don't have a problem with sending mails with winmail.dat from outlook. I'm receiving mails with winmail.dat in outlook which obviously undertands the TNEF format.

  • Hi William,

    Thanks for your updates.

    I'd like to confirm the following things:
    1. Does this issue only occur when the external users send emails to the Office 365 user?
    2. Have you already finished hybrid deployment with Office 365?
    3. Please provide us with more detailed information about your on-premises environment?

    Best Regards,
    Edward Qu

  • Hi Edward,

    1. The issue only occurs when receiving mails from one specific external domain to this office 365 user.

    2. The hybrid deployment is live and several users are migrated.

    3. The on-premise environment is an old SBS 2003 system, but it still does a better job on handling these emails apparently.


    William Acke

  • Hi William,

    I appreciate your patience and cooperation working on this issue.

    Please go to Office 365 Exchange Admin Center > protection > connection filter > add the IP of the specific domain to the IP Allow list, and then test again.

    Edward Qu

  • How do I know what their mail server IP address is?

  • Hi William,

    Sorry for the delayed response.

    Please try contacting the IT of the specific mail server.

    Edward Qu