The Zix Secure Message Portals do not support messages encoded in RTF (Rich Text Format), a byproducts of Microsoft’s TNEF encapsulation process. Messages directed to a Zix Message Center, which have been encapsulated using TNEF, can present formatting issues in the message content and present any attachments as winmail.dat files.
By using PowerShell to disable TNEF when sending to outside domains, the default Microsoft Office 365 behavior of encapsulating MAPI messages properties will be suppressed.
The full Microsoft KB article can be found here: https://support.microsoft.com/en-us/kb/2487954
Summary Solution
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Disable TNEF encoding for all messages sent to all domains outside your organization Disable TNEF encoding for all messages sent to all domains outside your organization.
To configure remote domains, you must use Windows PowerShell.
To learn how to install and configure Windows PowerShell and connect to the service, see Use Windows PowerShell in Exchange Online.
The following settings are available with the TNEFEnabled parameter on the Set-RemoteDomain cmdlet:
• $true TNEF is used for all messages sent to the destination domain.
• $false TNEF is never used for any messages sent to the destination domain.
• $null This is the default setting. TNEF messages aren't specifically allowed or prevented for recipients in the destination domain. Whether TNEF messages are sent to recipients in the remote domain depends on the specific setting on the mail contact or mail user, or the setting specified by the sender in Outlook.
Run the following command:
Set-RemoteDomain Default -TNEFEnabled $false