@Override
  public String toString() {
    return MoreObjects.toStringHelper(this)
        .add("folders", folders.size())
        .add("messages", messages.size())
        .toString();
  }
A Wrapper for all the possible objects that can be returned by a mail exporter.
portability-types-common/src/main/java/org/datatransferproject/types/common/models/mail/MailContainerResource.java