Callout 1 Messages remain on the server. Outlook connects to the server, and you can read or delete your messages.
Callout 2 Messages are downloaded to a local data file called a Personal Folders file (.pst). Outlook connects to the local .pst file so that you can read and delete your messages.
Callout 3 Copies of messages are downloaded and synchronized with messages on the e-mail server. Microsoft Exchange Server accounts provide this functionality by creating a local Offline Folder file (.ost). This file is used for working offline when you can't connect or don't want to connect to the e-mail server. This file is also used by the Cached Exchange Mode feature in Outlook 2003.
E-mail account types differ in how e-mail messages are saved and synchronized with the e-mail server. For example, by default POP3 e-mail accounts delete e-mail messages from the e-mail server when downloaded into Outlook. However, you can customize how e-mail messages are retrieved and saved. Other e-mail accounts, such as an Exchange Server e-mail account, save all e-mail messages on the e-mail server by default. You can configure an Exchange Server e-mail account to keep a copy of e-mail messages on your computer that allows you to read e-mail messages when not connected to the Exchange Server. For information regarding how different e-mail accounts handle e-mail message, see the following: