This flash screencast gives you step-by-step instructions for configuring Outlook 2003 as a POP client for your FlexBox Managed Email Account
The information you need to setup Outlook 2003 is as follows
Username = user@example.com
Incoming Server = flexbox.mxtoolbox.com
Outgoing Server = smtp.mxtoolbox.com
More Settings
My outgoing server requires Authentication, user same information as incoming
Incoming Server uses SSL
Outgoing Server uses SSL
Keep copy of messages on Server (this will use storage quota, but will allow you to use the web based client as well as Outlook)