When you attempt to Test and Enable a mailbox in Dynamics 365, the following alert is logged:
ActivityId: <GUID>
>Exception : Unhandled Exception: Microsoft.Crm.CrmSecurityException: SecLib::AccessCheckEx failed. Returned hr = -2147187962, ObjectID: <GUID>, OwnerId: <GUID>, OwnerIdType: 8 and CallingUser: <GUID>. ObjectTypeCode: 4120, objectBusinessUnitId: <GUID>, AccessRights: WriteAccess at Microsoft.Crm.BusinessEntities.SecurityLibrary.AccessCheckEx2(ExecutionContext context, SecurityPrincipal principal, SecurityPrincipal ownerPrincipal, Guid objectId, Int32 objectTypeCode, Guid objectBusinessUnitId, AccessRights rights) at Microsoft.Crm.Asynchronous.EmailConnector.ExchangeSyncUtility.HasExchangeSyncIdMappingAccess(Guid userId, Int32 userIdTypeCode, IACTProviderContext orgContext) at Microsoft.Crm.Asynchronous.EmailConnector.ExchangeSyncWorker.PerformPreMailboxSyncChecks() at Microsoft.Crm.Asynchronous.EmailConnector.ExchangeS..."