i'm not too familiar with linux (yet), but with Windows there is a php.ini file that, depending on how you're sending emails, needs to be edited, and it has a unix entry too for sendmail_path =
; For Unix only. You may supply arguments as well (default: "sendmail -t -i").
;sendmail_path =
maybe you need to tell it where sendmail or whatever you're using to send mail is located?
hth.