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Check your Sent folder for any messages that you know you didn’t send.
If you have a hybrid topology, the solution might also be related to the on-premises mail transfer agent.
To do this, open the mailbox in a web browser that supports only the light version of Outlook Web App, and use the following procedure: Remove all recipients Solution 4: Update forwarding rules to remove incorrect addresses Sometimes the NDR is caused by a recipient that the user didn't directly send mail to.
It could be that the intended recipient has either a forwarding rule or a delegate set up for their mailbox.
Solution 4: Make sure your email account wasn't compromised Did you send the original message at all?
If not, it’s possible that a spammer or someone else inappropriately used your email address when they sent the message.