From this page you can allow/disallow access from the control panel to web-based mail interface (Webmail), and specify the settings for handling incoming mail addressed to the users not existing at this domain.
For the mail to nonexistent users, you can choose any of the following options:
If you wish to use a web-interface for working with mailboxes in this domain, select the WebMail checkbox. You will then be able to access the web interface by specifying the URL: webmail.domain_name.tld in the web browser, or by clicking the WebMail icon on the Mail Name Properties screen.
Note: activation/deactivation of webmail for the domain involves modifications in DNS zone, therefore, there is actually a certain delay present when deactivating webmail due to the speed of DNS registration mechanisms.
Click OK to submit the changes.