ExternalDirectoryObjectID missing on EXO mailbox after msol user was accidentally hard deleted - Restoring the mailbox back.
As an Office365 admin you might have come across situations wherein the Msol-User was accidentally hard deleted, which resulted in the mailbox moving to Inactive state and not reconnecting back. Additionally, a new mail-user or cloud only mailbox is found for the user in EXO. In a Hybrid De ployment with Multi forest sync or an Exchange resource forest environment where AD attributes & Custom attributes are synced from one forest and exchange attributes from the other to collectively provision a single user in O365, And if that msol-user is hard deleted in the organisation, the "ExternalDirectoryObjectID" of EXO mailbox that is linked to "Objectid" attribute of MSOL user in Azure AD is permanently lost resulting in the mailbox moving to Inactive state which can also be called as an Orphaned mailbox. Due to this the mailbox never re-connects even after AAD-C syncs a new object to O365 a it generates a new ExternalDirectoryObjectID which does not ma