Outlook sends winmail.dat to iphones and Mac instead of ICS calendar

We moved from an on-premise Exchange 2007 to Office 365. Now when we send an ics calendar, the iphone and MACs receive it as a winmail.dat file. The receiving email accounts are mixed, Yahoo, Gmail, earthlink, etc., all with the same problem. Before with Exchange 2007 we did not have this problem. We tested from Outlook 2010, 2013, same results.

We accessed the webmail portion of each receiving account and the attachment comes in fine. We also disabled TNEF in the registry, setup Outlook to send in plain text and configured the contacts to send in plain text as well. If we send from the Outlook Web App with Office 365 the attachment shows up fine. So the combination of Outlook and Mac/iphone is having the problems. By the way, the we tried with an Exchange account on the phones and those accounts receive them fine, just the other Gmail, Earthlink, Yahoo accounts don't.

Any suggestions?
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Vasil Michev (MVP)Commented:
You need to do this server side. Check for example here: https://support.microsoft.com/en-us/kb/2487954
NRTCFAuthor Commented:
thank you for the suggestion, that solution will not work since we have to create contacts every time and implement the solution. Are there any other suggestions?
Vasil Michev (MVP)Commented:
The solution will work just fine if you read the article to the end :)

You can control this on a per-domain basis, so you can easily adjust the setting for every contact in partner domain. Or even for *all* domains via:

Set-RemoteDomain Default -TNEFEnabled $false

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