Please refer to this link to configure your Outlook Express to send and receive your webmail.

http://www.3essentials.com/ticket/flash/outlook_express_pop.html

1) You plan to have access to your messages in Outlook Express as well as in the webmail interface. This is useful if you are on the road and away from your PC and still want to have access to your email. Please note that keeping the email in the webmail interface draws on the email quota (space designated per user, usually in "X" amount of Megabyte (MB) space). For instance, if your user has 50MB allotted to store email in the webmail interface and they go over the 50MB, any incoming emails are rejected. It's important to know how much space you have and if it's enough.

2) You plan on ONLY using Outlook Express to send/receive your email messages. In this case you will need to select "Delete Messages off Server" from within the Outlook account. This will keep the folders in the webmail interface empty so that you never unknowingly go over your quota and stop receiving emails. Please review your Outlook Express documentation or help menu item for this feature.