Office 365 user gets error 5.1.11 bounce back emails randomly

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Office 365 users gets error code 5.1.11 bounce back emails randomly.
Status code: 550 5.1.11

The recipient email address is a LegacyExchangeDN address, which isn’t used by the Office 365 service. You might see this error if you’ve migrated your organization’s email from on-premises to the cloud, or if your organization has a hybrid configuration and you synchronize your on-premises directory with Office 365. If clearing the recipient Auto-Complete List from the user’s Outlook or Outlook on the web doesn’t solve the problem, try to clear the related LegacyExchangeDN address from your on-premises Active Directory. Then synchronize the directory again.

Did removed auto-complete entry several times and still has issues.

Why this is happening and what is the fix?
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Most Valuable Expert 2015
Distinguished Expert 2018
Commented:
Are users replying to older messages? Clearing the autocomplete entries doesnt help in this scenario, instead you will have to populate the LegacyExchangeDN attribute. Read here: http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/A_9650-NDRs-and-the-legacyExchangeDN.html

Author

Commented:
I do have X500 added to the account in exchange online. Do I have to add in AD and where? Or have to edit the one exchange online and how?
Most Valuable Expert 2015
Distinguished Expert 2018

Commented:
Question is do you have a X500 entry corresponding to the LegacyExchangeDN for which the NDRs are being generated? You can reconstruct the address from the NDR entry, and you can use Get-Recipient to confirm whether it exists in O365.
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Author

Commented:
Can you list the steps how to do that?
Most Valuable Expert 2015
Distinguished Expert 2018

Commented:
Did you check the article I linked to above? It explains how to reconstruct the LegacyExchangeDN based on the information from the NDR message.

Author

Commented:
Yes but not to understand how to do it. Please help
Most Valuable Expert 2015
Distinguished Expert 2018

Commented:
Can you post the full NDR message?

Author

Commented:
Do i have to modify the record that starts with IMCEAEX and add to the user account? Does it affect anything to have more than one x500 records?
Most Valuable Expert 2015
Distinguished Expert 2018

Commented:
You need to "transform" this into the LegacyExchangeDN, then check whether this value is assigned to the user (or any user in your tenant).

Author

Commented:
How do I check that is assigned to that or other user?
Most Valuable Expert 2015
Distinguished Expert 2018

Commented:
Get-Recipient value_of_LegacyExchangeDN

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Author

Commented:
So connect to office 365 exchange online tenant using powershell and run this command?
Most Valuable Expert 2015
Distinguished Expert 2018

Commented:
Yes. Here's one of mine for example:

Get-Recipient "/o=ExchangeLabs/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=890dd294006d47779a6886992c4c13d2-shared1"

Name      RecipientType
----      -------------
sharednew UserMailbox

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