Spoofing the From address

[Dennis Putnam]

I have recently migrated from Mandriva to CentOS and am have the return 
of an old problem. My ISP requires that I go through their reply to send 
mail. That is all working with the exception of the generated from 
address. They require that I have a valid FDQN in the from response. 
This was all working with my Mandriva main.cf but CentOS apparently has 
a newer version of Postfix (6.2.2) and the config files are not 100% 
compatible. I had to manually create the equivalent settings based on 
the old main.cf. That seemed straightforward enough however, the from 
response is being rejected by my ISP because the domain on the from 
response is being picked up from /etc/hosts (which is a fake local 
domain) rather than the 'myorigin' parameter in main.cf. I cannot find 
how I fixed this on the old OS but obviously I am missing some parameter 
somewhere that sets the from FDQN to what my ISP requires.

Can someone point me in the right direction? Thanks.