A new Outlook Data File (.pst) is created by default in Outlook 2010 when you add a new POP3 account
Summary
Microsoft Office Outlook 2010 introduces changes to accounts that help you keep email messages better organized when those email messages come from multiple accounts. By default, Outlook 2010 automatically adds a new Outlook Data File (.pst) when you add a new POP3 account. However, earlier versions of Outlook let you select an existing Personal Folders file as the default delivery location. This could result in email messages from different accounts being delivered to the same Inbox.
More information
This new functionality lets you easily determine through which account an email message was delivered. The new functionality also lets you select through which account an outgoing email message is sent. And this includes email messages that are replies or that are forwarded.
Although this new functionality is the default when you create a new account, you can manually configure a new Internet Email account that uses an existing Outlook Data File (.pst).
How to manually configure accounts to use an existing Outlook Data File
To create a new account and have that account use an existing Outlook Data File (.pst), follow these steps:
On the File tab, select Info, select Account Settings, and then select Account settings.
In the Account Settings dialog box, on the E-mail tab, select New.
Select E-mail Account, and then select Next.
Select Internet E-mail, and then select Next.
Under Internet E-mail Settings, complete the form with the necessary information for the new account.
Under Deliver new messages to, select Existing Outlook Data File, and then select Browse.
Locate and then select the existing Outlook Data File (.pst).
Select OK.
Select Next, and then select Finish.
If you are starting Outlook 2010 for the first time, when you create an Internet email account, follow these steps:
When you start Outlook 2010, the Add New Account, dialog box appears.
Select Manually configure server settings or additional server types, and then select Next.
Select Internet E-mail, and then select Next.
Under Internet E-mail Settings, complete the form with the necessary information for the new account.
Under Deliver new messages to, select Existing Outlook Data File, and then select Browse.
Locate and then select the existing Outlook Data File (.pst).
Select OK.
Select Next, and then select Finish.
How to change the default delivery location
If the account is already configured to use the default settings, the default delivery location can be changed. To do this, access Account Settings, and then use the Change Folder option for the POP account.
To change the Outlook Data File (.pst) that an existing account uses, follow these steps:
On the File tab, select Info, select Account Settings, and then select Account settings.
In the Account Settings dialog box, on the E-mail tab, select the existing account that you want to change.
Select Change Folder, and then select the Outlook Data File (.pst) that you want to deliver new email messages to.
Select OK, and then select Close.