Forum Discussion
Exchange Hybrid Deployment single forest multiple email domains
Hi Abdul. In a hybrid configuration, it is best practice to have your on-premises UPN's set to match the SMTP addresses. If the UPN is set to a domain.local suffix, then you will see these accounts syncing up to O365 with the roommb1@domain.onmicrosoft.com as you are seeing now. I would strongly advise changing the on-prem UPN's to be the correct domain suffix.
You don't have to migrate DL's. These will be taken care of with your hybrid coexistence setup when you get to this.
Speaking of the Hybrid, it is worth pointing out that once you run your HCW, it will modify your email address policy to include an alias of username@domain.mail.onmicrosoft.com for each user who is targeted by and email address policy. This alias is important, and any accounts excluded from email address policies will need to have this added manually before they can successfully be migrated,
Once you have completed your mailbox migrations to O365, it is recommended to retain an Exchange Management server to manage any attributes which still have their SOA on-premises, and also to act as SMTP relay for software and devices.