reject sender with a different domain of mine

[David Brachet [Permanent Link]]

Hi all,

I have a problem: everybody can send a mail from my local network. For example, my domain is my_domain.com, and i can send a mail if my mail address is @toto.fr. I would like that a user on my local network can send a mail only if his mail address is 
toto@my_domain.com, and for toto AT toto DOT fr, the user must authenticate for example.

What is the rule i must add? I try several, but no result. Thanks