Buddypress send email notification only if user is not logged in [closed]

One thing you can do is to filter $email_to and return empty string if the recipient is logged in. This way wp_mail() will fail to send the message and return false. Add the following to theme functions.php or to bp-custom.php file:

add_filter('messages_notification_new_message_to', 'disable_loggedin_email_notification');
function disable_loggedin_email_notification($email_to) {
    $user = get_user_by('email',$email_to);
    if (bp_has_members("type=online&include=$user->ID")) {
        $email_to = '';
    }
    return $email_to;
}

EDIT: A possible solution for your case with the plugin you use is to get all the users who have that e-mail and check if any is online by passing that list to bp_has_members() function:

add_filter('messages_notification_new_message_to', 'disable_loggedin_email_notification');

function disable_loggedin_email_notification($email_to) {
    $users = get_users(array(
        'search' => $email_to
    ));
    $ids = array();
    foreach ($users as $user) {
        $ids[] = $user->ID;
    }
    $ids = implode(',', $ids);
    if (bp_has_members("type=online&include=$ids")) {
        $email_to = '';
    }
    return $email_to;
}