static boolean isValidIfPresent(@Nullable String email) {
    return isEmpty(email) || isValidEmail(email);
  }
@Test
  public void various_examples_of_unusual_but_valid_emails() {
    assertThat(isValidIfPresent("info@sonarsource.com")).isTrue();
    assertThat(isValidIfPresent("guillaume.jambet+sonarsource-emailvalidatortest@gmail.com")).isTrue();
    assertThat(isValidIfPresent("webmaster@kiné-beauté.fr")).isTrue();
    assertThat(isValidIfPresent("Chuck Norris <coup-de-pied-retourné@chucknorris.com>")).isTrue();
    assertThat(isValidIfPresent("\"Fred Bloggs\"@example.com")).isTrue();
    assertThat(isValidIfPresent("pipo@127.0.0.1")).isTrue();
    assertThat(isValidIfPresent("admin@admin")).isTrue();
  }