We are offboarding mailboxes from O365 to on-premises Exchange. We have a couple of mailboxes that have migrated successfully, but at completion have been configured as Linked Mailboxes in on-premises Exchange. The only thing I've found different between the accounts that have migrated correctly and the one's that haven't is that the X500 address in the proxyaddress attribute is incorrect. My assumption is that because this is incorrect, the reconfiguration step assumes the mailbox is located in another Exchange Organisation and configures the on-premises mailbox as a Linked Mailbox. Does this sound like a reasonable assumption, or should I be looking somewhere else as to why it's being configured as a Linked Mailbox?
I'm looking to go through on-premises AD and make sure all the accounts are configured correctly before migrating more mailboxes.
what does your on-prem topology look like? Do you have an AF/RF setup? Or just a single AD forest with Exchange in it? In the latter case, I would find it highly suspicious is a MBX is moved back and shows up as a linked mailbox.
The customer just has a single forest with Exchange in it, but it's possible that there has been a few acquisitions and mergers over the years which have lead to some variation in the AD account configuration.