Default Routing sets the behaviour of mail sent to you not addressed to a specific mailbox. This is only available if you have a Value host Pro or above, or a Value Host with the POP box upgrade.
You can choose one of three settings:
Bounce all mail not sent to POP3 boxes on the account: This will bounce any messages not sent to a mailbox you have. Recommended setting to avoid unwanted spam.
Send to a POP3 mailbox: You can select the mail sent to one of your POP 3 mailboxes.
Send to email address: Send the mail to an external email address.