Filtering Mails

[chino sili adnu edu ph]

Hi folks,

I have a mailing list everybody mydomain com which is for exclusive use
of mydomain.com members.   How do I configure sendmail such that only 
members of mydomain.com or mailserver.mydomain.com are allowed to post 
messages to the later.

I have heard about milter, is there any readily available script that 
would help me do what I've describe above. 

I am using RedHat 8 and the sendmail package included.

Thanks.

Carlo