Online mode When the Outlook profile is configured in Online Mode Outlook will create a direct connection to the Information Store of Exchange.
This file is also known as the offline data file which can be recognized easily when looking at the extension which is OST.
By default the file is stored in The advantage of Cached Exchange Mode is that mailbox content is also available when you don’t have a connection to Exchange, excluded the new items which haven’t been cached.
When connected to Exchange messages will arrive with a slight delay in the mailbox.
This is caused by Outlook which will check for new messages every 30 seconds by default.
A disadvantage of Cached Exchange Mode that the OST file is only cached locally.
When a user is working on another workstation a new OST file will be generated if not available.Starting from Outlook 2010 Microsoft recommends to use Cached Exchange Mode.When the autodiscover functionality of Exchange is being used you will see that the Cached Exchange Mode will be enabled by default. When using Outlook 2010 by default both the user mailbox and additional mailboxes are added to the cache. It is possible to add Public Folder favorites to the cache but in this case you will need to enable the option manually or via a GPO.As mentioned earlier Outlook will use an Offline Address Book (OAB).Initially a complete download is being performed of the address book.After that only incremental updates are downloaded from the address book.