UNIX for Advanced & Expert Users

We have a Spark Solaris 10 box running behind a Pix 501 firewall, we are running NAT and I have mapped access to it from the outside world. Anyway, we can receive mail no problem just can't send. I receive this message bounce:
The original message was received at Thu, 10 Jan 2008 17:04:29 -0600 (CST)
from scruffy.oakvalleynet.net [192.168.1.2]

----- The following addresses had permanent fatal errors -----
<richardl@aea16.k12.ia.us>

----- Transcript of session follows -----
550 5.1.2 <richardl@aea16.k12.ia.us>... Host unknown (Name server: mailhost.oakvalleynet.net: host not found)

If I do an NSLookup from the server it can find itself but sendmail doesn't seem to find it.
Have been pulling my hair out for quite some time over this, and and all advice would be much appreciated

It needs to find the mailhost, not itself. On the box that cannot send mail, "nslookup mailhost.oakvalleynet.net" needs to be able to find the ip address for that mailhost. It looks like that doesn't work.