GW8 is not connected to the Internet

1 - how to configure GW8 to use another email server to send emails to Internet/outside ?

2 - this GW8 server is working for mycompany.local domain, I want to configure this GW8 so that when it sends emails to outside of the GW system e.g to or to, then this GW8 change the sender address/domain from @mycom.local to

please help