mail() problem...

Hi, i wrote a helpdesk script, which can send the user emails, if requested. The helpdesk sends emails as "nobody@mydomain.com", but some servers don't accept "nobody" as the sender! do you know how i can change that? i don't mean the "From:" header, but the real sender header of the email... e.g.: "Received: from nobody by hostname.mydomain.com with local (Exim ......" Would i have to change the apache user to change this? I mean, there are so many scripts out there and they all have a proper "Received: from..." header, so i guess i simply overlooked s.th. Thanks for your help, Duncan .