Pre-Existing permissions which were granted before migrating the mailbox to office 365 works, But any new mailbox permissions given post the migration breaks in a hybrid scenario. For example, if we have three mailboxes Mbx1, Mbx2 & Mbx3. Consider Mbx1 has send as permission on Mbx2 and now Mbx2 is migrated to O365, in this case the permission works seamlessly. But if i try to add send as permission for Mbx3 on Mbx2 , it won’t work.
Exchange hybrid configurations do support the use of the Send-As, Receive-As, or Send on behalf of mailbox permissions, these permissions are only available when both the mailbox granting the permissions, and the mailbox receiving the permissions, are in the same realm. Any mailboxes that receive these permissions from another mailbox need to be moved at the same time as that mailbox. If a mailbox receives permissions from multiple mailboxes, that mailbox, and all of the mailboxes granting permissions to it, need to be moved at the same time and exist in the same realm of either on premises or Office 365 Exchange organizations.
Watch this video from 44:00 for more clarity.
Ganyboy 