code: func (dnsClient *DNSClientImpl) LookupMX(ctx context.Context, hostname string) ([]string, error) {
  	dnsType := dns.TypeMX
  	r, err := dnsClient.exchangeOne(ctx, hostname, dnsType)
  	if err != nil {
  		return nil, &DNSError{dnsType, hostname, err, -1}
  	}
  	if r.Rcode != dns.RcodeSuccess {
  		return nil, &DNSError{dnsType, hostname, nil, r.Rcode}
  	}
  
  	var results []string
  	for _, answer := range r.Answer {
  		if mx, ok := answer.(*dns.MX); ok {
  			results = append(results, mx.Mx)
  		}
  	}
  
  	return results, nil
  }
docs: // LookupMX sends a DNS query to find a MX record associated hostname and returns the
  // record target.