Sending email.....using mail() does not seem to work

Hi,
I am trying to use my php scripts to send email but am having difficulty....I used to use the unix 'sendmail' program when writing PERL and I also have it installed now on my machine.

When I try to use the mail() funtion although it does not give me an error it does not send the mail either....

Any ideas??

thanks

To add more info to that...
I am using a machine with php3 installed running apache..

in the php.ini file i have set the
'sendmail_path = /usr/lib/sendmail'

and if I run the phpinfo() function it sees the path correctly, it also gives the correct path to the php.ini file (if I change anything I see the changes in the result of phpinfo())

but still when I have a simple script that does nothing except send an email like this:
<?php
if (mail("david@21network.com", "Test", "test")) :
echo ("Sent");
endif;
?>

anyideas??? I am really stuck...
Dave