Hi @Kanta Prasad ,
Real issue revolves around the legacyExchangeDN transfer as X500 from Old mailbox to new mailbox and system not allowing me to add this address as a X500.
As far as I know, it's supported to transfer the LegacyExchangeDN from an INACTIVE mailbox to another mailbox as a X500. See: Restore an inactive mailbox:
Set-Mailbox <identity of target mailbox> -EmailAddresses @{Add="X500:<LegacyExchangeDN of inactive mailbox>"}
I was wondering why you want to keep the old mailbox alive? If you are intending to keep the old mailbox data, what about disabling it and then restore the content into the new maillbox, or exporting the old mails to PST files and then disable it?
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