If you are trying to clean up a large mailbox, see also the Mailbox Cleanup solution.
Create an Outlook Data File (.pst) to save your information
When you run Microsoft Outlook for the first time, the necessary data files are created automatically. However, sometimes you'll create additional data files. For example, archived items can be saved in another Outlook Data File (.pst). You might also keep different projects in their own Outlook Data File (.pst).
If you are using a Microsoft Exchange Server account, an Outlook Data File (.pst) is usually not necessary as your items are delivered to and saved on the computer that is running Exchange Server. However, Exchange accounts include functionality in Outlook to enable you to work with your messages even when a connection to the email server isn't available. This feature uses a special offline Outlook Data File (.ost) that is used to keep a synchronized copy of the Exchange account.
About Outlook Data Files (.pst and .ost)
Outlook Data Files (.pst and .ost) created by using Microsoft Outlook 2010 are saved on your computer in theDocuments\Outlook Files folder. If you are running Windows XP, these files are created in the My Documents\Outlook Filesfolder.
The primary differences between the two Outlook Data File types are as follows:
An Outlook Data File (.pst) is used for POP3 and IMAP email accounts. Additionally, when you want to create archives or backup files of the Outlook folders and items on your computer, such as Exchange accounts, you create and use an additional Outlook Data File (.pst).
An offline Outlook Data File (.ost) is used only when you have an Exchange account and chose to work offline or you use Cached Exchange Mode. It isn’t necessary to back up an offline Outlook Data File (.ost). A new .ost file can be created from the data saved on the computer is running Exchange Server.
Create an Outlook Data File (.pst)
On the Home tab, in the New group, click New Items, point to More Items, and then click Outlook Data File.
In the Create or Open Outlook Data File dialog box, in the File name box, type the name as you want it to appear in the Outlook Navigation Pane, and then click OK.
By default, data files are not password-protected. To add a password to your data file, under Password, type that password that you want to use in the Password and Verify Password text boxes.
IMPORTANT: Microsoft, your Internet service provider (ISP), or your email administrator don’t have access to your password, nor can they assist you with recovering the contents of the Outlook Data File (.pst) if you forget the password.