MessageTrace

Question

Hi All,

I am using an Exchange 2016 hybrid environment. All my users are created on-premises and migrated to Exchange Online.One of my users, let's say user1@contoso.com, sent an email to a distribution list (DL) DL1@contoso.com. This DL has 1,000 members and is an Exchange Online Dynamic DL.However, user2 received the email even though user2 is not a member of the DL. I suspect there might be a forwarding rule, mailbox forwarder, or some other rule that caused the email to be sent to user2.Please guide me on how to trace this.

Answer 1

Hi，@Rising Flight

Thanks for posting your question in the Microsoft Q&A forum.

According to your description, when an email is sent to the Dynamic DL, members who are not members of the Dynamic DL receive the email.

I have a couple more questions to confirm based on your question.

1.Is this a problem for anyone sending emails to Dynamic DL?

2.Is this email only forwarded to Users2?

You can try the following:

You can use Message Trace to see if messages are being forwarded by rules.

I set up the following rule to simulate your current problem.

1. Sign in to the Exchange Administration Center (EAC) in Office 365.
2. Navigate to Mail Flow and then Message Trace.
3. Set the recipient to user2@contoso.com.

If that email is forwarded by a rule, you can see a screenshot similar to the one below at Message events.

If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.