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Outlook 2016 Winmail.dat attachment replaces real attachment when using Outlook.com and Office 365

Anonymous
2016-12-08T17:30:56+00:00

When we send emails with attachments from Outlook 2016 on our Windows 10 computers using either an Outlook.com account or an Office 365 account with to users of Apple products, the real attachment is removed and replaced with a file called Winmail.dat.  Also, the email format is not HTML, despite settings in Outlook to the contrary.  The problem appears to exist across all Apple product lines, as it is evident for recipients using iPhones and iPads, even when using the Outlook mail client for iOS.  It also exists for recipeints using Macs, even when they are running Outlook for Mac OS.

This seems to be a recent development, as I know we have been able to previously send attachments to users of Apple products.  Insofar as the Outlook.com accounts are concerned, it is not critical to be able to send attachments.  However, we use Office 365 for work emails, and the ability to send attachments to users of any OS is a necessary feature.

I know this problem does not exist with Google Apps, and while I would prefer not to have to switch over to Google, unless this issue with Office 365 is fixed soon, we will have no choice but to make the switch.

Is anyone else having this problem?  Is there a solution?

Outlook | Windows | Classic Outlook for Windows | For home

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Anonymous
2016-12-10T10:54:56+00:00

Hi BOBSQUESTION,

Normally, this is caused by Transport Neutral Encapsulation Format (TNEF), which is affected by Rich Text Format (RTF) settings in Outlook.

If you are using office365 accounts, when sending mails to an external user whom is added as a mail contact, please do steps below with the administrator account:

1.    Connect to exchange online by using PowerShell

2.    Run commands:

Set-MailContact  ******@xxxx.com -UseMapiRichTextFormat never

Set-MailContact  -Identity ******@xxxx.com -UsePreferMessageFormat  $True

Get-MailContact  ******@xxxx.com |fl

3.    when finishing all steps, please let end user check the settings in their outlook client. Here are the steps:

click: file—options---mail, under message format option, choose to convert to HTML format.

After changing outlook settings, do not forget clear the auto-complete cache which may keeping the previous setting for a recipient.

If your situations are different, could you please provide information below for us to reproduce your environment?

1.    The version of outlook which has that problem

2.    Were all attachments affected? What kinds of attachments are being replaced?

3.    Let me know if you are a sender or a recipient.

4.    Do senders and recipients both use office 365 for business account? Do they use the same domain or different domains but in the same tenant?

5.    Please let me know the name of any other kinds of mail which were affected by this problem.

Best Regards,

Erick

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  1. Anonymous
    2016-12-13T03:43:06+00:00

    Hi David,

    I would like to ask some updates about the information mentioned above. 

    Best Regards,

    Ruel

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  2. Anonymous
    2016-12-09T17:28:14+00:00

    I ended up getting assistance for the Office 365 account from Microsoft Office 365 tech support.  They made a change in the TNEF setting within my Office 365 account using PowerShell, which is apparently the only way to address this issue.

    While the issue is fixed, I noted to the rep that this was not at all intuitive and added that there should be an easier way to address this -- e.g., using the Office 365 Admin feature.

    What the tech did appeared similar to the article that Ruel Mayoya linked to, although I can't say for sure it was identical, so I can't comment on whether or not that will work.

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  3. Anonymous
    2016-12-09T16:38:58+00:00

    This does not resolve the attachment being converted.  Assuming this only fixes the actual message?  Still need a resolution

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  4. Anonymous
    2016-12-09T16:31:10+00:00

    Hi David,

    I understand that when apple user receives an outbound message,  attachments become Winmail.dat instead of the original attachment. The reason is that the sending mail application user Rich Text format or Transport Neutral Encapsulation Format (TNEF).

    To prevent winmail.dat attachments , please follow this article to change the message format for:

     - external contacts

    -  that are sent to a specific domain.

    If the issue still occurs, please provide the following information:

    1. On Windows Outlook, try to send messages in HTML or plain text format. Then check if the attachments still change on Mac.

    2. Check under **File > Options > Message Format > When sending messages to rich text format to internet recipients:**set to Convert to HTML.

    1. The detailed Outlook version.

    Looking forward to your updates.

    Best Regards,

    Ruel

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