allow send email only from local domain

[supportadl]

Good afternoon, I need an hand to make a custom configuration on my Sendmail 8.13.8. I already configure it to allow relay only for authenticated users. But I want to increase sevurity and make also another limitation if it is possible: allow sending email only for local domain. For example, domain A.COM is locate in my server and can send email to everyone, domain B.COM is NOT locate on my server and I want to block access to my smtp server also if it uses authentication.
I need this to block possible spam send from an infected pc through my mail server using different email address.
This is my actual access file:

Connect: localhost.localdomain RELAY
Connect: localhost RELAY
Connect: 127.0.0.1 RELAY

Can anyone help me?
Thanks a lot.