Our office has been connected to the Internet via VSAT Broadband recently and we installed our mail server, but now there are some messages passing out our server to the external world (neither of the users come from our address list).

First make sure your mail server is not doing SMTP mail relay for everyone on the Internet. Your SMTP mail relay should only be for your users. Currently, if ALL your users and the mail server have public IP addresses to access the Internet, then you need to change all the users and mail server to private IP addresses and put the RF550VPN between your LAN and your WAN. Then setup virtual server and open the SMTP port for your mail server so you can send / receive mail over the Internet.