Hope you like it..

How to offboard Exchange Online Mailboxes to Exchange On-Prem using Exchange Hybrid Environment

December 22, 2024 at 11:07 am Leave a comment

Recently , we had a project to offboard Exchange Online Mailboxes to On-Prem Exchange. In this environment customer already have Entra Connect Server. I would like to share the steps we followed during the project so it could be referred by anybody who is having a similar requirements.

However , I am not going to list out any commands here as it is freely available in many web sites.

1. First we deployed Exchange On-Prem Server and completed all the configuration like DAG , Connector etc.
2. Then we deployed the Exchange hybrid Configurations with Centralized Mail Transport.
3. All the autodiscover,MX and other Exchange related DNS records were re-pointed to Exchange On-Prem Public IP’s.
4. Created a TestMailbox in Exchange On-Prem and tested the email flow.
   – Email sent to Exchange On-Prem Mailbox from Outside delivered successfully.
   – Email sent to ExchangeOnline Mailbox from outside bounced back with an error user not found in the organization.
   – Email tests sent from Exchange On-Prem to Exchange Online and vice versa failed.
   
5. After troubleshooting we found out the Exchange Online mailboxes were not visible on Exchange-On-Prem ECP. Further troubleshooting revealed that the AD user attribute for the AD synced users does not have Remote Routing Address.(e.g testdomain.mail.onmicrosoft.com)
   
6. So we decided to do a bulk update of all the users to populate their Remote Routing Address.(You must do a Delta AD Sync after this process). Thereafter , we were able to see all the Exchange Online Mailboxes in the Exchange On-Prem ECP.
   
7. Tested all the email scenarios and confirmed everything is working fine.
   
8. The other obstacle we faced is that , when an email sent from Exchange Online Mailboxes to outside world the relaying was denied by the Exchange On-Prem server.Even though , we believed the default FrontEnd receive connector in Exchange On-Prem had all the requirement for this, we ended up creating a new Receive Connector and added all the Exchange Online IP’s to allow the email relay and resume the e-mail flow.

Hope it helps some one who is struggling to find a proper article for similar scenario. In case , if you want to reach out to me for detailed steps like commands used for bulk import/export/AD properties update , please feel free to reach out to me.

Entry filed under: Exchange and O365. Tags: azure, centralized-mail-transport, exchange, exchange 2019, exchange hybrid, exchange-online, microsoft, migration, offboarding, powershell.