One global mail address – howto?

I have a network with about 20 computers.

I’m trying to do that whenever user send mail, the receiver can see the mail like it send from one address.

example:
instead of the mail send from user1@mydomain.com, user2@ …
it will send from office@mydomain.com.

any idea how can i do this on exchange 2003 enterprise server?

Thanks a-lot