how to allow send email just to users of my linux box ?
 

I again everyone, i have a new problem, maybe i'm so crazy and this could no be posible to resolv, but anyway i'm going to ask you.
I have an email server over linux with sendmail, i have like 400 users, currently i have been detected that someone on internet has been sent email with some of my email users, this is he or she catch an user of my domain and send email sign with our information user@mydomain.com this bring me troubles because i receive some emails from diverse postmasters that say that user@mydomain.com has been sent a virus or pornography, and that's not true.

The question is:
Is there any command, configuration, program, etc, that could allow send email just to my user's linux box ? i mean that anyone else can not send email no matter if they use any user with my domain

or

How can i block users that doesn't exists on my linux box ? I mean, if a user doesn't exists, that he or she receive a message like: user@mydomain.com needs to be a valid account anyway user@mydomain.com can't send email.

or something like that.

sorry for my english, i known is not to good but i hope you can help me.

Regards.

Can you give a specific example (using madeup names)? I am not sure what you are trying to accomplish from your description.

o.k. dkloes, i'll try to explain what do i neeed.

I found that everybody that has a conection with my company ISP can send email masquerading the email user, for example, my email account is prueba@mydomain.com where de account is prueba, if a decide change de email address test@mydomain.com instead of prueba@mydomain.com i still sending email anyway, i known that's because the user is the same in preferences, the username still be prueba but the email account changed.
The question is : is there any method to avoid this, a mean if a user change his email address he doesn''t still sending messages no matter if the has the same user account ? In there any program or method that can authentificate the email address ? i mean that verify if it is a valid email address permit it send email if not reject .

Thanks for your help.