Under the Delivery section of the Advanced tab, Outlook 2003 helps you define how to manage old emails on the mail server.

Outlook 2003 downloads your emails from the server when your click the Send/Receive button, and in effect deletes them from the server after successful downloads. Within the advanced tab you can customize Outlook's email deletion behavior. Outlook 2003 gives you 3 options.

1. Tick this box if: you want to remove from server after X days checkbox is checked, Outlook will automatically delete the copies of emails that are that many days old.
2. Tick this box if: you want to remove from server when you deleted from 'Deleted Items'.

Mac OS X Mail:
How can I set Mail to delete messages from the server after retreiving them?

Highlight the account you want to modify; then go to:

Under the Advanced tab when editing a mail account - tick the box to 'Remove copy of message from server after retrieving a message'

Mail -> Preferences -> Accounts -> Advanced