mail() -> How to change the sender

Hi, all my scripts on my server, which use the mail() function, get the sender: nobody@domain.com where domain is the localdomain of the server and nobody is the user under which apache is running. So, now i recently saw an error in the logs, that some servers don't accept emails coming from a "nobody@" scheme. How can i change the sender, so that i can use s.th. else? Will i have to change the apache user to get rid of the "nobody"? I am not talking about the "From: " or "Reply To:" parameters, but the actual "Received: from nobody by domain.com with local (Exim ....) ....." line in the header. My server runs with RedHat 7.3 & latest apache 1.x & php versions. Thanks in advance, Duncan