This hook allows the default selection in the subscriber drop down on the Send Email page to be changed. By default ‘registered’ which selects Registered Subscribers. Other options are ‘all’, ‘public’, ‘confirmed’, ‘unconfirmed’ and ‘all_users’. Respectively these are All Registered Users and Public Subscribers, all Public Subscribers, only Confirmed Public Subscribers, only Unconfirmed Public Subscribers and All Registered Users.

function my_preferred_default( ) {
    // return 'all' or 'public' or 'confirmed' or 'unconfirmed' or 'all_users'
    return 'public';
}
add_filter( 's2_subscriber_dropdown_default', 'my_preferred_default' );