You will need to talk to whoever manages your edge routing or firewall device to find out if they have mapped a public IP address to your SMTP server. It is possible that by default the server just uses network address translation (NAT) to gain access to the internet and send emails. About the only way to see what IP address your server is using to send is to send yourself a message on GMail, Yahoo or some similar account and look at the message headers. The IP address of the sending server should be in the message header.
In the IT trenches? So am I – read my IT-Trenches blog