When you start typing a name in the Outlook To: field, the resolution behavior depends on your Cached Exchange Mode settings and the offline address book (OAB).
How it works:
- Cached Mode (default): Outlook searches the local OAB first. The OAB is downloaded from Exchange and updated on a schedule (default: once per 24 hours)
- Online Mode: Every lookup queries the Exchange server directly using the GAL
- Autocomplete cache: The NK2 file (pre-2010) or .dat stream stores previously-used addresses — these resolve first, before either OAB or GAL
Common issues:
- New users don't appear in lookups until the OAB regenerates and the client downloads it
- Stale autocomplete entries point to moved/deleted mailboxes — clear with
Ctrl+clickon the suggestion or empty the cache
Admin tip: Force OAB generation on Exchange with: Update-OfflineAddressBook "\Default Offline Address Book"
Modern context: Exchange Online handles OAB distribution automatically. For hybrid environments, ensure your OAB generation schedule aligns with directory sync intervals. In pure Exchange Online, the "GAL" is backed by Entra ID — so user provisioning in Azure AD means near-instant availability in Outlook lookups.