can't send mail (php5,apache2,ubuntu9)

[dthscyth]

I created a mail script but it won't send any mail. I changed my php.ini into something like this:

; For Win32 only.
; http://php.net/smtp
SMTP = smtp.vivax.com
; http://php.net/smtp-port
smtp_port = 25

which I personally think is correct. Do I have to install sendmail or ssmtp for this mail script to run? I'm using php5, apache2 and ubuntu server. Thanks.