Exchange 2000 sending from wrong server

Hi,
We are experiencing a problem where all of our outgoing email say they are comming from address 65.x.x.34. 65.x.x.34 is the natted address of our domain controller where the inside address is 10.0.0.3. The exchange server is at 65.x.x.37 with the inside address of 10.0.0.4. Exchange is only installed on the one server so we can’t understand why the mail headers say 65.x.x.34. It should say 65.x.x.37 as we are experiencing lots of problems with people who use reverse lookups to block spam. Any ideas?
Thanks
Pete