public class ModelBuilder
{
    IList<IModelBuilder> _loggers;

    ModelBuilder(IList<ModelBuilder> processors) {
      _processors = processors;
    }

    EmailModel Log(EmailMetadata emailMetadata) {
      EmailModel model = new EmailModel();
      Log(emailMetadata.Attachments, model);
    }

    void Log(IEnumerable<Attachment> attachments, EmailModel emailModel) {
      foreach(var attachment in attachments) {
        foreach(var processor in _processors) {
          processor.Log(attachment, emailModel);
        }
      }
    }
  }
}