Description
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 fix the situation
To correct this mail flow issue, we recommend that you refer to the methods that are provided in this Microsoft article and summerised below.
Keep Exchange Online mailbox
- Disconnect the on-premise mailbox:
Disable-Mailbox <mailbox identity>2. Enable the on-premises user as a remote mailbox:
Enable-RemoteMailbox <mailbox identity> -RemoteRoutingAddress "user@contoso.mail.onmicrosoft.com"Those are the basic steps to enable the online mailbox and remove the on-premise mailbox. More advanced options including keeping the LegacyExchangeDN or transferring data to exchange online can be found in the Microsoft article.
Keep the Exchange On-Premise mailbox
Naturally, this method is a little more complicated. Follow this article for instructions