When sending outbound through Email Security .cloud, an email sent to one or more recipients outside your organization fails to arrive to the intended recipients.

- No Non Delivery Report (NDR)
- Within the Client portal, Email Track and Trace does not show a delivery attempt to the target recipient

 

The ReturnPath/Envelope Sender of the email for the DL is a Null Sender by default in Office 365. 
The "Sender" in your Office 365 report is simply a "Body From" and is not the SMTP sending address. 
As a result Email Security.cloud servers cannot verify that the email is legitimately from a customer, so they respond with a "trying to use me as a relay" message.

1. Within Office 365, on the settings for the DL, set a manager for the DL that is an address within a domain protected by Email delivery . cloud
2. Using a Powershell command enter  "set -ReportToManagerEnabled $true"

Microsoft articles related to this issue.
http://support.microsoft.com/kb/2723654