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Were having a problem where sending emails with attachments from Outlook (connected to exchange) works fine, receiving these emails works fine, however if someone then forwards these on to someone else, then the attachment is replaced by winmail.dat. This happens when forwarding to both internal users and external.

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up vote 0 down vote accepted

Make sure that your clients are not using Rich Text for the email format. This is the default if you use Word as your email editor, but it can be switched to HTML format instead.
Here is the Microsoft explanation for this.
Here is the search on Microsoft support if you have other environments not covered in the first one.

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Thanks, I had thought it was a Rich text problem, and I'd changed the clients that had the issue to using HTML, and it went away for a while, then came back. Will investigate this route some more. – Sam Oct 20 '09 at 8:29
You're welcome. I am not an expert on Exchange, but I believe the Exchange server can also cause the conversion to RTF. You might want to look at your server too. Good luck. – Scott Lundberg Oct 20 '09 at 13:16

You can tweak the properties on your default remote domain (iirc only available with PowerShell) to force content conversion preferences.

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