Let me preface this by saying I hate Exchange..
Now that that's out of the way... So I was clearing out some stuff in ADUC and deleted an OU that still had some disabled users in it (whoops!) who are in domainB but use Exchange in domainA as linked mailbox users. I managed to get their accounts back but I'm not having any luck getting their disconnected mailboxes back linked to their accounts in domainB.
I've used the GUI and get the following error:
The user account "domainA\user" specified to be the master account for this linked mailbox is already the master account of another linked mailbox "domainA/P&S Insurance/P&S Users/userfirst userlast". Please specify a different user account.
I've also tried the following command in Exchange Management Shell:
$cred = Get-Credential
Connect-Mailbox -Identityuser -Database "Mailbox Database"...