code: func (m *MailerImpl) resetAndError(err error) error {
  	if err == io.EOF {
  		return err
  	}
  	if err2 := m.client.Reset(); err2 != nil {
  		return fmt.Errorf("%s (also, on sending RSET: %s)", err, err2)
  	}
  	return err
  }
docs: // resetAndError resets the current mail transaction and then returns its
  // argument as an error. If the reset command also errors, it combines both
  // errors and returns them. Without this we would get `nested MAIL command`.
  // https://github.com/letsencrypt/boulder/issues/3191