Troubleshoot email delivery in Exchange on Microsoft Online

I had a problem the other day where I noticed that of all the emails I sent to the same recipient outside my orgianization, some were delivered and some were not. They were not in Junk mail and I did not receive any non-delivery reports.

I find the Microsoft Online user interface to be a little… puzzling … to use a nice word. I have learned that I cannot expect to find any links to the admin interface but have to remember to use my bookmarked url to

Exchange Admin Center (EAC)

Yesterday I learned that I can also access something called the Exchange Admin Center (EAC). From there it is possible to, among other things, run a mail trace for any message sent the last 7 days.

1. Go to (if you, like me, have a separate admin account, make sure you sign in with this)

2. Click Mail flow and then Message trace

3. Fill out the Sender and Recipient information and hit search. Double click any message to see details.

My undelivered emails

From the EAC Message trace I can see that the messages that were not delivered are marked as “Spam” and action set to “Use high risk delivery pool”. It is then set to “Send” and details say “Message transferred from: To_DefaultOpportunisticTLS”.

My best guess is that it means the email is currently in waiting mode and will be delivered or fail some time in the future… I have contacted Microsoft support and will update this post when I get an answer.


Source: Microsoft help article: Troubleshoot email delivery using the Exchange Online message trace tool


  1. Unfortunately I did not get a clear response to this.
    Microsoft checked their logs etc and concluded that the problem was on the recipients side. Recipient is also on office365/micrsoftonline, but they could not trouble shoot without them opening a support ticket also which was never done.

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