Microsoft Outlook changes the email of one particular recipient to their old email address after I have sent my email

Question

I work with a person outside of my organization on several projects. They changed their name and therefore their email address has changed. Whenever I send them an email, I receive a delivery failed notification. The delivery failed notification clearly shows the emails failed to go to the old address despite having sent it to the new address. Outlook is changing the recipient's email after I send the email.

I have deleted this person from my contacts and the automatically populated list of address suggestions, I have updated my global offline address list., I have tried to send her emails from the app on the phone and from my computer with the same result.

Answer 1

Hi @Anne Marken ,

Are you receiving 5.1.x message delivery errors? Can you provide specific error code?

From your description, I understand that you have done some troubleshooting steps, there are some other causes of this error:

• Contact the recipient and ask them if they've configured an email rule that forwards incoming email messages from you to another destination. Their rule could have tried to send a copy of your message to a bad email address.
• An invalid legacy domain name (DN) exists for the recipient's mailbox Active Directory Domain Service.

For more information, see Fix email delivery issues for error code 5.1.1 through 5.1.20 in Exchange Online.

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