Email messages which match the fully qualified name of your machine will be accepted by Sendmail. Others will be either bounced back or forwarded.
You may want to accept email messages for different domains. One situation is when you register with an Internet provider and he gives you one subdomain of his own. You start using it, and later, you request your own domain. You still want to accept the old one, because many people know you by this address.
You are allowed to enter as many lines as you need here. Use the Add button to grow the dialog.