Sendmail Configuration

I use Red Hat Linux Server 7.2 in my lab. One is set to use domain puth.com.kh and the other is set to use domain mylab.edu.kh. All mail server of these machines have the name mail.puth.com.kh and mail.mylab.edu.kh. I use POP3 and SMTP. At domain mylab.edu.kh, I do like the following:
$vi /home/choth/.fectmailrc
poll mail.puth.com.kh with POP3
user “choth” there with password “mypassword” is choth here
How/What should I configure the server machine mail.puth.com.kh to be able to deliver all mails mylab.edu.kh if the outside users send to *@mylab.edu.kh? I would prefer to receiving your configurations by hand and please tell me each steps too. Many thanks.