[CentOS] sendmail security-with example

> In above example u can see in the 1st example the
> sender(xyz) is not real user of my domain , still he
> is able to sendmail to my real users (abhi).

You know that I can still connect to your server and send an email with 
address abhi at domain.com to abhi at domain.com right? Do you really want to 
prevent xyz at domain.com?