Where did I say type Return-Path?
mail($recipient,$subject,$message,$headers,"-f$returnAddress");
It's simply -freturn@mycompany.com
You don't write it like a header.
Oh, I think you need PHP4.0.5
AFAIK = As Far As I Know
RTFM = Read The F'ing Manual - you really should do that sometime - it has explainations and examples 🙂