In migrating from Notes to Office 365, the outlook shared address book or GAL only showed users who have active accounts in Office 365. As you can imagine this creates problems for users who have been migrated to Office365 as they are unable to find addresses of users whose mailboxes still reside in notes. All users were populated with mail and proxyaddresses attributes in AD and Directory Synchronization was running for several weeks. As a workaround, Office 365 users were setup to use an LDAP address book in outlook that pointed back to notes but this creates outlook cache issues and confusion when creating meeting requests as free busy is not displayed when the meeting organizer selects an invitee from the LDAP address book. Another alternative is to create contacts in Office 365 for notes users with a separate namespace, activate all the accounts and have them forward back to notes using newly created contacts.
The way to avoid this problem all-together though is to populate the target address attribute with Office 365 email address for all users in addition to mail and proxyaddresses attributes in local Active Directory!