mail() problem help.....

Hi, Sir: I develop my own webmail on my website . A few days ago, I join this php mailling list. After that, I found I could not send any mail to this mailling list. I guess.... Because I sent mail with PHP's mail() function, It seems like every mail's header shows "nobody " on the first line of Return-path : ------------------------------------------------------------------ Return-Path: <nobody@fonn.fongming.idv.tw> ^^^^^^^^^^ and... Received: (from nobody@localhost) ^^^^^^^^^^^^^^ ------------------------------------------------------------------ I guess this is the reason why my mail was always rejected by php mailling list. How Can I replace the "nobody " to a normal user name ? Thanks for any helps..... Fongming from Taiwan.