Please help with mail function

Hello all, I have a big problem with the mail function in a php script. If I run this script with root access, the mail function is working. If I try to run it like http://www.i-s-u.nl/test.php it won't work..... What is happening, what can I do??? Below the little script: <? mail("rolf@berkenbosch.nl", "onderwerp", "test test test", "info@leensweb.nl"); ?> Thanks, Rolf