How To Perform An Extended Message Trace In Office 365
Troubleshooting Office365 email issues with Microsoft Technical Support normally involves sending them the results from an extended message trace which includes events and routing details in the report. As these traces can take a while, it is a good idea to generate them as soon as you raise a support ticket, but it is not obvious how to do them..  Firstly, you need to know either the Message ID (you can get this from a standard trace) or the sender / recipient email address.

To start a message trace, login to the Office365 portal and select 'Admin Centers' followed by 'Exchange'. Then select 'mail flow' and 'message trace'

The extended message trace options are hidden.  You need to select a 'Custom' date range and at least 8 days ago

Next tick 'Include message events and routing details with report' and add your email address to the 'Notification email address'

Next, either enter a Message ID or the sender's email address

Enter the email address into 'check names' then click the check names button, followed by OK

Click 'search in the bottom right corner' to submit your request

At the top of the message trace tab you will now see a link called 'View pending or completed traces'

Once completed, you will receive an email which you can then forward on to Microsoft Support.