When using the On Demand Migration (ODM) Hybrid module to migrate accounts from the source local Active Directory to the target Active Directory, you may encounter the following error:
This user's on-premises mailbox hasn't been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed.
If you have users in the source local Active Directory environment that previously had on-premise Exchange accounts in the source, then those users likely have the following attributes populated in the source Active Directory:
msexchmailboxguid
msexchrecipientdisplaytype
MsExchRecipientTypeDetails
Currently, the ODM Hybrid module will sync these attribute values to the target local Active Directory, which in turn will sync these attribute values to Office 365 via Azure AD Connect. However, syncing the "msexchmailboxguid" attribute value in particular will prevent a target Office 365 mailbox from being created by the ODM mail migration.