You can configure the mail settings for domains hosted on your sphinx shell account using the hosting accounts page in the Customer Control Panel. The control panel allows you to create aliases within your domain. These aliases can either forward mail to another address, or place mail in mailbox. Mailboxes can be accessed using either POP3 or IMAP from any email client. The settings needed to receive mail from POP3 mailboxes, or to send email, are documented on the Mail Settings page.
If an email arrives to an alias that you have not defined, normally it will be bounced as a non-existent address. Alternatively, if you define an alias called _default_ (starts and ends with an underscore), as either a forwarder or a mailbox, that action will be used for all otherwise unknown addresses. If you do this, be prepared to receive some unsolicited email—some spammers will send email to randomly generated addresses within registered domains.