When using the singleton design pattern, do other methods need to use synchronized keyword to ensure thread safety?

I want to ensure that the following class is thread-safe, should I use the synchronized keyword for other methods? Or use a thread-safe data structure to store Email. What should I do?

public class RecycleStation {
    private static volatile RecycleStation uniqueInstance;
    private static List<Email> recycleEmailList ;

    private RecycleStation() {
        recycleEmailList = new ArrayList<>();
    }

    public static RecycleStation getInstance() {
        if (uniqueInstance == null) {
            synchronized (RecycleStation.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new RecycleStation();
                }
            }
        }
        return uniqueInstance;
    }

    public void RecycleEmail(Email email) {
        recycleEmailList.add(email);
    }

    public void deleteEmail(Email email) {
        recycleEmailList.remove(email);
    }

    public void clear() {
        recycleEmailList.clear();
    }

}