not sure if this can even be done, but can someone basicly spoof a mail header? my isp just called me and said that they had around 100 complaints from people saying that they recieved spam from one of my ip's (a box mananged by one of our developers). this is odd becuase at the firewall only port 80 is open to this box, and smtp(which is installed) is shutdown and is not allowing mail relay anyway???

i'm going through log files now, but any insights would be helpful.