Outlook 2003's Mail Setup options give you a centralized access to configure your email accounts, Outlook's Send/Receive settings, data file management (PST files) options, and Outlook's behavior with regards to dial-up connections.
To open Outlook 2003's Mail Setup options, go to Tools > Options from Outlook's main window, and select the Mail Setup tab from the Options dialog.
The E-mail Accounts button lets you customize existing email accounts in Outlook 2003. The E-mail Accounts dialog that opens is also accessible by going to Tools > E-mail Accounts from Outlook's main window.
Outlook 2003's Send/Receive Options determine how Outlook will check for new emails, how often, and let you create custom groups of email accounts.
More information on Outlook 2003's Send/Receive Options…
Outlook 2003's Data Files options let you manage personal PST files settings like location, file names, as well as letting you optimize your PST files
More information on Outlook 2003's Data Files Options…
Outlook 2003's Dial-up options determine how Outlook manages (connects to or disconnects from) dial-up connections you might have; these settings are not important if you use a high-speed, always-on connection to the Internet.
If you are using a dial-up connection, make sure to keep the default choice, "Warn before switching dial-up connection": you may otherwise find Outlook 2003 interfering with your other online activities.
Automatically dial during a background Send/Receive (unchecked by default) can also be an annoyance if Outlook is set to periodically check for new emails on the server, as this would mean regularly instantiating a new connection if you are offline.
Hang up when finished with a manual Send/Receive is enabled by default; it can be disabled to prevent Outlook 2003 from interfering with your dial-up connection.