Internet down at the site office. Train journey, client floor, power cut with a dying hotspot. Offline is where the two Outlooks differ most - one carries your mailbox, the other carries a recent slice of it.
Classic Outlook was built for a world of dial-up and travel: Cached Exchange Mode keeps a full copy of your mailbox in an OST file, so reading, searching, replying and organising work completely offline, then sync when a connection returns. The new Outlook is cloud first: it keeps a limited local cache of recent mail for viewing, and many actions simply wait for the internet. This page compares what actually works in each app with no connection, so field staff and low connectivity offices choose with open eyes.
Example panel - cache size and behaviour vary by build and settings, test your own machine offline.
In classic Outlook with Cached Exchange Mode, the OST file mirrors your mailbox. By default it can hold a year or everything, controlled by the Mail to keep offline slider. Offline, you can read any synced mail, open attachments, search with the local index, move items between folders, build new mail that sits in the Outbox, and even work inside PST archives. When the connection returns, changes reconcile with the server. The weaknesses are equally real: OST files grow large, can corrupt, and a stolen laptop carries a mailbox copy protected only by the Windows sign-in.
The new app stores a cache of recent mail locally so the app opens and shows recent messages offline, and Microsoft has been expanding offline abilities in stages - reading and composing basics first. But it is a window onto the cloud mailbox, not a mirror of it. Older mail beyond the cache is not there. Full mailbox search leans on the server. Features that need live services - add-ins, smart features, some calendar actions - pause until reconnect. For a desk with fibre and rare outages this is invisible. For a person whose workday is half offline, it is a daily friction that no setting removes today.
Offline pain rarely comes from mail reading - it comes from the meeting address you cannot open, the contact number stored only in Outlook, or the attachment the client expects you to forward. Classic carries calendar, contacts and mail together in the local store. In the new app, offline calendar and contacts access is also cache shaped and thinner. A practical habit for both worlds: truly critical contacts also live in your phone, and truly critical documents also live in a synced folder. Relying on any mail app as the only offline copy is a risk, whichever Outlook you choose.
Vendor pages describe offline features at their best. Your office needs them at their worst - floor three, monsoon evening, shared hotspot. So test physically: disconnect, then perform the five tasks your role does most. Time it. Note what is missing, not what is present. Classic will pass almost everything and look old doing it. The new app will pass the recent-mail basics and fail the deep archive tasks. Both results are useful. Choose per role: desk roles on fibre can enjoy the new app, field roles keep classic until the cache story changes.
One more offline-adjacent truth: when the connection is slow, classic's local copy means most clicks are instant and only changes sync. The new app's cloud first design can feel slower on weak links even while online, because more actions round trip to the server. On a healthy connection the difference vanishes. Offices on struggling last mile connections should pilot during their worst network hours, not their best - an app chosen at noon on empty fibre may be judged very differently at seven in the evening on a loaded link.
See both Outlook apps side by side in the full video - every difference shown on a real PC, step by step.
Watch Full VideoOnly mail inside its local cache, which focuses on recent items. Older mail generally needs a connection. Classic Outlook with Cached Exchange Mode keeps the synced mailbox fully available offline.
For synced mail, calendar, contacts and local PST files, yes - reading, searching, organising and composing all work, and changes sync when you reconnect.
Classic Outlook today, for roles that spend serious time offline or with old mail. New Outlook suits always connected desks and its offline abilities are still growing in stages.