Hi,
You can select which account you want to Send from, look at the following
help topic from Outlook 2003 Help:
Specify which e-mail account to use to send a message
If you have set up multiple e-mail accounts (e-mail account: The server
name, user name, password, and e-mail address used by Outlook to connect to
an e-mail service. You create the e-mail account in Outlook by using
information provided by your administrator or Internet service provider
(ISP).) in Microsoft Outlook, you can specify which account to use when
sending a message.
In the message, click Accounts.
Click the account you want to use.
Notes
By default, e-mail is sent by using the account specified as the default in
the E-Mail Accounts Wizard (Tools, E-mail Accounts command). For example,
your default account may be your work e-mail account, but you can specify to
send a message from another e-mail account, such as your personal Internet
service provider (ISP) (ISP: A business that provides access to the Internet
for such things as electronic mail, chat rooms, or use of the World Wide Web.
Some ISPs are multinational, offering access in many locations, while others
are limited to a specific region.) account.
Sending messages from your ISP account may require you to be connected to
that provider in order to receive permission to use the e-mail server.
Contact your network administrator or ISP for more information.
Hope this helps