By default, classic Outlook stores mailboxes as a copy on the computer (Cached Exchange Mode). For large mailboxes, low disk space, terminal servers or problems with shared mailboxes, switching to online mode helps. Data is then loaded directly from the server.
Shared mailboxes only online (recommended for shared mailbox issues)
Open File › Account Settings › Account Settings ….
Select your own account and click Change ….
Open More Settings › Advanced tab.
Clear the Download shared folders check box.
Finish with Apply › OK › Next › Finish and restart Outlook.
Entire mailbox online
Open File › Account Settings › Account Settings ….
Select your own account and click Change ….
Under Offline Settings, clear Use Cached Exchange Mode.
Click Next › Finish and restart Outlook.
Good to know
In online mode, Outlook only works with an internet connection. This is not ideal for laptops often used offline.
Search may be somewhat slower in online mode.
New Outlook doesn't need this setting. There, Settings › General › Offline controls whether data is stored locally.