Staged migration to O365, NDR from non-migrated to migrated users


I am in the middle of a staged migration from our On Premise Exchange 2010 to Exchange Online. Everything is smooth so far and I successfully started migrating some internal users. However, non migrated users receive an NDR when trying to reach newly migrated users. I found the culprit, which is the autocomplete. Typing the complete address or deleting the autocomplete entry resolve my issue.  Is there is any way to force the update dynamically those entries ?

Thanks !
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Jamie McKillopIT ManagerCommented:

How did you do the migration? The issue you are running into is related to the legacyExchangeDN value (see my article here - Depending on the method and tools you are using to migrate, the legacyExchangeDN may not have been migrated over as an X500 address. To resolve this, you will need to export all the legacyExchangeDN values from your on-premise mailboxes and import them into the O365 mailboxes as X500 addresses.


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maxalarieAuthor Commented:
Hi Jamie,

Thanks for your help, You are right, the LegacyExchangeDN have not been migrated as an X500 address. After resolving this, mails are not bouncing back anymore.
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