Attachments is seen as WINMAIL.DAT from certain receivers

Honored colleagues and experts.

I know this question sounds pretty common, but I can't figure this out, exactly!

I am the administrator of a Small Business Server 2011 network.
All clients run Windows 7 with Office 2010.

For some strange reason, one user, apparently only this one, is having problems by sending mails with attached PDF files to one certain external recipient. This recipient is seing attached files as the famous WINMAIL.DAT.

If this user, from the same physical computer is using his GMAIL, he is able to send to this recipient with success.
Also every single, other user within this SBS domain, is able to send attached docs to the recipient.

I first belived, that this was an obvious problem with settings or security with the recipient, but I am no longer sure.

Can anyone give me a hint?
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Ben HartCommented:
Do you have any attachment filtering going on?
Will SzymkowskiSenior Solution ArchitectCommented:
This is usually due to having Rich Text setup on the client. Also depending on the attachment it could be getting stripped out by AV or Anti-spam appliance.

This is most likely a Outlook Editor setting specifically set to Rich Text. Have them change it to HTML and test again.

Ben HartCommented:
Also verify whether or not your user is sending Rich text or HTML.
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OhmitAuthor Commented:
I can verify that Outlook editor it is set to HTML
In your Exchange Powershell management, use this:

Set-RemoteDomain Default -TNEFEnabled $false

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OhmitAuthor Commented:
Hello Kimputer

Can you please explain what this does?
Also I am pretty sure, that this effect the entire domain, but it seems like only one user has this problem.
It doesn't matter that this command is for the whole domain, as this will solve the issue for the user AND every other user in the future that might come across this problem. There are NO adverse effects at all. This winmail.dat nonsense started with older Exchange versions (for me, it bothered me from 2003 onwards, even up to the newest 2013 and Office 365, but possible earlier versions were affected as well). I have always solved it, and no user will ever complain.
Just start the Exchange Management Shell and input this command.
Read more about it here:
It will remove the need for you to ever check winmail.dat issues, as now, even if the user makes a mistake, it's solved at the server side.
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