Recently I was doing an email migration from Exchange On-premise to Office365, and the customer's environment is not suitable to use Dirsync.

This means that I will need to export the X500 address from my local Active Directory and import it into Office365.

Here is the brief steps if you are facing this issue:

Export the LegacyExchangeDN and mail field in your AD:
1. In your DC server, run Command Prompt
2. Run the command:
csvde -f c:\sample.csv -l legacyexchangedn,mail

Now, with that .csv file, make sure that it has a “mail” and “legacyexchangedn” under the heading. See my attached sample file: sample CSV with LegacyExchangeDN

Import the legacyexchangedn/X500 into Office365/Exchange Online using PowerShell:

1. Connect to Exchange Online. using PowerShell
2. Run the command:

Import-CSV C:\sample.csv | foreach {

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$user=Get-Mailbox $_.mail
$user.EmailAddresses+=”X500:”+$_.legacyExchangeDN
Set-Mailbox $_.mail -EmailAddresses $user.EmailAddresses

}

To check if the address has been added:
Get-Mailbox $user | FL emailaddresses