Outlook saves data in a personal folder file, which uses the .pst extension. A typical PST file contains data from Calendar, Mail, Contacts, Tasks and your personal folders. If you're on an Exchange Server, the data is in an Exchange Server mailbox that lives on a server instead of the local computer. The only way to access this data store is by connecting to the server.
Outlook also creates the Offline Folder File with an .ost extension. This stores your data for working when your computer isn't connected to the Internet or network. The server could be down or you might be away from the office and its network. The Exchange Server data file will continue receiving messages and managing files as it always does. You can create e-mail messages, new folders and file e-mail messages into folders while offline. Any work done offline goes into the OST file, which synchronizes with the Exchange Server the next time it connects.
Outlook 2007 relies on Cached Exchange Mode, which provides a copy of your mailbox on your computer and synchronizes with the Exchange Server. Cached Exchange Mode always uses an OST file whether a connection is active or not. The nice thing about this feature is that you could lose a connection to the server and continue working in Outlook as if nothing happened.
It's possible to have a PST file even with an Exchange Server account. However, any changes to the PST file aren't synchronized with Exchange.
Working with at least two computers is common today. If you want the second computer to have the Outlook folders and data on the first computer, just copy the PST file to the second computer in Outlook using the following steps:
- Open Account Settings from the Tool menu.
- Select the Data Files tab.
- Click Add.
- Leave the Types of storage selection as is.
- Click OK to locate the PST file.
- Select the file.
- Click OK.
- Give it a name in the Create Microsoft Personal Folders window.
- Click OK.
- Close Account Settings and the new personal folders file appears in the Navigation Pane.
You can also import a PST file using Import and Export from the File menu.