Deleting messages on server when read on a POP3 account in Outlook 2003 versions:
1. Open E-mail Accounts:
2. Select View or change existing e-mail accounts:
3. Select your POP3 email account and click Change:
4. Click More Settings:
5. On Advanced tab, unselect Leave a copy of the messages on the server, click OK to save settings:
Now all new email messages are deleted from the server as soon as Outlook has received them.