This problem is very urgent and difficult.
We set up a sbs 2003 server, with a new domain. The server is alone as DC. Everything is working correctly except the mailing. Internally we can send and receive mails. Exchange is set up to receive internet mail with pop3 and to send them via the smtp server of the internet provider. When sending from a client computer(in the domain) we receive an answer from the local system administrator saying that the mail address does not exist. This mail address is working correctly on a workstation(not in the domain), or with outlook express.
We tried everything suggested in experts-exchange site (instants questions). But nothing works.