What is a default email address?
A default email address is where all unspecified mail sent to your domain name goes. For example: Your default e-mail address has been defined as moe@threestooges.com and you also have an e-mail address set up for curly@threestooges.com.
Any addresses other than curly@threestooges.com (like larry@threestooges.com, for example.) will then be automatically forwarded to moe@threestooges.com.