Loop over the email line by line. If you encounter the header you don't want, don't include it...
<pseudo code/>
$message="";
while(looping over email)
{
if (!ereg("^To:",$this_line) || !ereg("^From:",$this_line))
{
$message .= $this_line>
}
}
OR
if it's plain text only, the message part is separated from the headers by a blank line (\n\n), search for that.