After migrated the user mailboxes from earlier version of Microsoft Exchange to the latest version of MS Exchange server, users are able to access their mailboxes in OWA (Outlook Web App) but they may get the below error while accessing outlook in all the system:
This error occurs if multiple accounts have same LegacyExchangDN.
To isolate the issue, you have to check the logs in Client Access Server (Program Files\Microsoft\Exchange Server\V15\Logging\RPC Client Access)
Open the log file then do a search with the user log-in account to get the logs related to the particular user and you may get logs for the more than one email address with same Legacy Exchange DN value.
Please copy the Legacy Exchange DN [/O=Test/OU=Exchange Administrative Group/cn=Recipients/cn=TestUser] then use the PowerShell command below to check the mailboxes related to the same Legacy Exchange DN.
Get-Recipient ‘O=Test/OU=Exchange Administrative Group/cn=Recipients/cn=TestUser’
You have to check X500 (Proxy Address) value in those mailboxes and update unique value in all the mailboxes.
Wait for 15 to 20 minutes and again try the same PowerShell command to confirm whether user has unique Legacy Exchange DN value. Finally now users can successfully access their mailboxes by using outlook.