From the CPanel online
documentation:
Any e-mail that is sent to an unknown account
at your domain name, such as unknown@yourdomain.com,
gets automatically rerouted to your default e-mail
account. All web site accounts are automatically
assigned a default e-mail address - user@yourdomain.com
- which you can change, if required.
1. Click on the
Default Address
link in the Mail Menu area.
2. Click on the
Set Default Address
link.
3. Enter the complete e-mail address of the new
default in the field next to your web site name
drop-down list.
Note: You can enter
:blackhole:
to throw away all incoming mail, or
:fail:
to bounce the e-mail back to the sender.
4. Click on the

button.
Your new default e-mail address has now been set.