How to recover when a mailbox exists in both Exchange Online and on-premises
In a Microsoft Exchange Server hybrid deployment, a user may have a mailbox in both Exchange Online and an Exchange on-premises organization. This isn’t a desired state for a hybrid organization because it will create mail flow issues. In this case, messages will be delivered to the mailbox that corresponds to the location of the sender. If the sender is located in your on-premises organization, messages will be delivered to the on-premises mailbox. If the sender is located in your Exchange Online tenant, messages will be delivered to the Exchange Online mailbox.
How to improve the situation
To correct this mail flow issue, we recommend that you refer to the methods that are provided in this article. Other possible options use recovery methods that are not guaranteed to work. As Office 365 continues to evolve and new features are added, additional options may be possible. This article will be updated to reflect additional corrective methods as they become available.
Scenario 1: Keep Exchange Online mailbox
This scenario would be most applicable if the user mailbox was previously migrated to Exchange Online, and somehow the old mailbox was reconnected or a new mailbox was provisioned on-premises. To use this method, follow these steps:
Save the on-premises mailbox information to a file, such as "SMTP addresses," "Exchange attributes," and so on.
Set the PowerShell Format enumeration limit to "unlimited" to make sure that no attribute values are truncated. For example:
$formatenumerationlimit = -1 Get-Mailbox “mailbox identity” | fl > mailboxinfo.txt
Disconnect the on-premises mailbox:
Disable-Mailbox “mailbox identity”
Enable the on-premises user as a remote mailbox:
Enable-RemoteMailbox “user identity” -RemoteRoutingAddress email@example.com
Restore any custom proxy addresses and any other Exchange attributes that were stripped when the mailbox was disabled (compare to the Get-Mailbox command from step 1).
(Optional) Stamp the Exchange Online GUID on the remote mailbox (required if you ever want to offboard the mailbox back to on-premises).
Set-RemoteMailbox “user identity” -ExchangeGuid “Exchange guid value of Exchange Online mailbox”
Restore the contents of the disconnected mailbox to Exchange Online.
$cred = Get-Credential New-MailboxRestoreRequest -RemoteHostName mail.contoso.com -RemoteCredential $cred -SourceStoreMailbox “exchange guid of disconnected mailbox” -TargetMailbox “exchange guid of cloud mailbox” -RemoteDatabaseGuid “guid of on-premises database” -RemoteRestoreType DisconnectedMailbox
Scenario 2: Remove Exchange Online mailbox data
The mailbox information in Office 365 may no longer be needed. In this case, see this Exchange Team Blog article for more information about how to remove the Exchange Online mailbox information completely.