@Moto
It's quite possible, and normal, for the SMTP gateway to be seperate from the OWA. And, in fact you're probably hitting a front end server not the actual Exchange server itself. (Putting an Exchange server, or any private mail store on the internet is asking for trouble...)
Two easy options. Ask your network administrator for the SMTP server address, or send yourself an email from inside and you may be able to discover the gateway name by inspecting the mail headers (provided they use the same SMTP gateway for incoming and outgoing messages).
