Declarated fifth param ( )at http://www.php.net/manual/en/function.mail.php 
is apsent in mail.c from php-4.0.4pl1.tar.gz
;(

bool mail (string to, string subject, string message [, string additional_headers [, string additional_parameters]])

string additional_parameters - don't have any code in mail.c
And this example from http://www.php.net/manual/en/function.mail.php 
about using Return-path: dos'nt work!

Example 3. Sending mail with extra headers and setting an additional command line parameter.

mail("nobody@aol.com", "the subject", $message,
     "From: webmaster@$SERVER_NAME", "-fwebmaster@$SERVERNAME");

such code give us error 
=======
Warning: Wrong parameter count for mail() 
=======
bcos, as i say, in mail.c don't present any code about 5'th param :(

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Sorry for my bad english.