code: func (t *MailTarget) Open(errWriter io.Writer) error {
  	t.Filter.Init()
  	if t.Host == "" {
  		return errors.New("MailTarget.Host must be specified")
  	}
  	if t.Username == "" {
  		return errors.New("MailTarget.Username must be specified")
  	}
  	if t.Subject == "" {
  		return errors.New("MailTarget.Subject must be specified")
  	}
  	if t.Sender == "" {
  		return errors.New("MailTarget.Sender must be specified")
  	}
  	if len(t.Recipients) == 0 {
  		return errors.New("MailTarget.Recipients must be specified")
  	}
  	if t.BufferSize < 0 {
  		return errors.New("MailTarget.BufferSize must be no less than 0")
  	}
  	t.entries = make(chan *Entry, t.BufferSize)
  
  	go t.sendMessages(errWriter)
  
  	return nil
  }
docs: // Open prepares MailTarget for processing log messages.