apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/listener/ConfigPublishListener.java/ConfigPublishListener/buildEmail
class ConfigPublishListener:    private Email buildEmail(Env env, ReleaseHistoryBO releaseHistory, int operation) {
      switch (operation) {
        case ReleaseOperation.GRAY_RELEASE: {
          return grayPublishEmailBuilder.build(env, releaseHistory);
        }
        case ReleaseOperation.NORMAL_RELEASE: {
          return normalPublishEmailBuilder.build(env, releaseHistory);
        }
        case ReleaseOperation.ROLLBACK: {
          return rollbackEmailBuilder.build(env, releaseHistory);
        }
        case ReleaseOperation.GRAY_RELEASE_MERGE_TO_MASTER: {
          return mergeEmailBuilder.build(env, releaseHistory);
        }
        default:
          return null;
      }
    }