Assuming that the script is PHP, you are missing a field. The mail function arguments are as follows:
bool mail( $to, $subject, $body, $headers)
The bool that is returned is true if the e-mail was successfully passed on to the sendmail daemon. The to field is the e-mail address where the e-mail is to be sent, $subject and $body should be self-explanatory, and $headers is where you enter the various headers. Each header needs to be seperated with "\r\n". This is where you should be specifying the sender e-mail address.
nobody@yourserver.tld is the default user for your web server application. This automatically gets added as the return-path, since that is the user that originated the e-mail. However, if a From: is entered in the fourth parameter, it will override the return-path and that will not be visible unless the recipient manually views the headers.