How do I delete email from the server?

This would depend on wish application you are using to manage your emails on the web.

By default the Plesk and Cpanel use the Horde application.

1. Log into your Mailbox by visiting the www.namhost.com/webmail
2. Click on the Inbox on the left hand navigation.
3. Tick the mail that you wish to delete or select all by using the drop down selection at the top of the Inbox.
4. Just click the delete link at the top and your done deleting your emails.

The better option is to have your email client delete old emails periodically.