Reconnecting Disconnected Mailbox to Another Account

“The operation cannot be performed because this mailbox was already reconnected to an existing user.”
ID no: c1034ad6
Exchange System Manager

This is a problem we run into quite a bit and amazingly almost everywhere you search on the internet they will tell you to remove the exchange attributes on the new account then reconnect the old disconnected mailbox to the account, exmerge it, remove the Exchange attributes again, and finally reconnect the new disconnected mailbox.

A much easier solution is to simply change the legacyExchangeDN of the user to something else temporarily and then you can reconnect the disconnected mailbox to any temporary AD account you want. Once the mailbox is connected, change the legacyExchangeDN of the temporary AD account to something else and finally change the legacyExchangeDN of the user back to the original.

Now some might kick and scream because technically you could generate a NDR if someone sends a message to the user during these 45 seconds using a cached address in Outlook. If that is the case, which it probably is, you could prevent the NDRs by adding the legacyExchangeDN attribute as an X.500 address for the user account.

To do this create a Custom Address for the user and enter the legacyExchangeDN as the address with the address type being X500

Once you’ve changed the temporary AD account's legacyExchangeDN to something other than the original and added the legacyExchangeDN back to the user you can remove the X500 address.