code: @Nullable
      private String extractDomain(String email) {
          int indexAt = email.indexOf(AT_SYMBOL) + 1;
          if (indexAt == 0) {
              return null;
          }
          String domain = email.substring(indexAt);
          if (domain.isEmpty()) {
              return null;
          }
          return domain;
      }
docs: Extracts the domain part from the email
  
  @param email to parse
  @return the domain String if found, an empty String otherwise