There is no SMTP proxy itself in the rack untangle. However, it seems that you want to allow only e-mail from domain the@toto.com ...
First, I look at the configuration of the multifunction (? - What is it exactly?) To relay only users@toto.com. This is usually fairly simple to do with most mail servers.
Secondly, use a rule of firewall to allow the evacuation only outgoing SMTP mail server IP and a second rule to deny outbound SMTP (and log) to all guests. In this scenario, traffic from the mail server is the rule of firewall first and e-mail will be allowed to pass. All other hosts will be the match the second rule and will be denied the SMTP transport.