apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/component/config/PortalConfig.java/PortalConfig/emailSender
class PortalConfig:  public String emailSender() {
    String value = getValue("email.sender", "");
    if (Strings.isNullOrEmpty(value)) {
      value = emailConfigUser();
    }
    return value;
  }