Source:
http://maisonbisson.com/blog/post/12939/using-wordpress-with-external-smtp-server/

Find in /wp-includes/pluggable.php the line that says $phpMailer->isMail();

Changed that to $phpMailer->isSMTP();

Now edit /wp-includes/class-phpmailer.php. Change the settings to whatever you wat.

...
public $Host = 'mailserver.domain.com';
...
public $SMTPAuth = 'true'; //(or false)
...
public $Username = 'myusername';
...
public $Password = 'mypassword';
...