MicrosoftHello Mirela_Buru The_Exchange_Team , we have a hybrid setup with one tenant and Exchange 2013 servers. Now we have introduced Exchange 2019 and setup hybrid with another tenant. Users are placed in 2 separate OUs as per UPN suffix and synced to the two tenants through separate AADC servers. Then there is a load balancer. Exchange 2013 and 2019 servers are added to it for http connections. When Exchange 2019 servers are for smtp it breaks relay from applications. Any idea why? We chose to ignore that (please don’t ask why) and attempted migrating mailboxes, but it fails. Based on your last comment, is it because Exchange Online also reaches MRSproxy of Exchange 2013 servers through load balancer and it is unable to proxy to Exchange 2019. The error is “MigrationRecipientNotFoundExeption: a recipient wasn’t found for <email address> on the target. Create a recipient of the appropriate type on the target and try again.” Microsoft support is at it for days. One final question, you have emphasised that a mail enabled user needs to exist on Office 365, is this only visible only through PowerShell or can be validated through EAC Office 365 as well? From previous hybrid migrations, I have not seen any thing like “on-premises” under mailboxes in Office 365 EAC similar to “Office 365” mentioned for migrates mailboxes in on-premises EAC.
