code: public boolean isAccountValid(String email) throws HelloSignException {
          if (email == null || email.isEmpty()) {
              return false;
          }
          Account account = new Account(httpClient.withAuth(auth).withPostField(Account.ACCOUNT_EMAIL_ADDRESS, email)
                  .post(BASE_URI + VALIDATE_ACCOUNT_URI).asJson());
          return (account.hasEmail() && email.equalsIgnoreCase(account.getEmail()));
      }
docs: Returns true if an account exists with the provided email address. Note
  this is limited to the visibility of the currently authenticated user.
  
  @param email String email address
  @return true if the account exists, false otherwise
  @throws HelloSignException Thrown if there's a problem communicating with
  the HelloSign API.