add_action('phpmailer_init', 'configure_smtp');
function configure_smtp(PHPMailer $phpmailer)
{
    $phpmailer->isSMTP(); //switch to smtp

    $phpmailer->Host = 'smtp.example.com';
    $phpmailer->SMTPAuth = true;
    $phpmailer->Port = 465;
    $phpmailer->Username = 'admin@example.com';
    $phpmailer->Password = 'mypassword';

}

add_action('phpmailer_init', 'configure_smtp', 10, 2);
function configure_smtp($smtp_id, PHPMailer $phpmailer)
{
    $user_details = get_option("smtp_user_".$smtp_id);
    $phpmailer->isSMTP(); //switch to smtp
    //rest of the code
}

do_action("phpmailer_init", $smtp_id);

wp_mail($to, etc);

Catchable fatal error:  Argument 2 passed to configure_smtp() must be an instance of PHPMailer