Try add the following to config/user_setup.php. Be sure to substitute values of your own where applicable.

// Your mail server
ini_set("SMTP", "mail.YourDomain.com");

// Please specify an SMTP Number 25 and 8889 are valid SMTP Ports.
ini_set("smtp_port", "25");

// Please specify the return address to use
ini_set('sendmail_from', 'ValidEmailAccount@YourDomain.com');