Tracing Threatening Email to an IP Address

[ka4731 () aol com]

My friend and I received a threatening email from an anonymous yahoo account.  We believe this may have come from one 
of my friend's coworkers because the header information was traced back as far as the IP address of the Cisco Pix 
firewall within my friend's company network.  All emails I receive from my friend have the IP address of his computer 
within his network.   Why is it that the anonymous email header doesn't tie me to a computer or work station just like 
my friend but only shows the IP of the firewall?