checker.go

package main

import (
	"bufio"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
	"os"
)

var (
	filename = "domains.txt"
)

type timedResponse struct {
	*http.Response
}

func getMXRecord(domain string) {
	mx, err := net.LookupMX(domain)
	if err != nil {
		fmt.Printf("Error looking up mx for %s - %v\n", domain, err)
	} else {
		for m := range mx {
			fmt.Printf("%s: %s\n", domain, mx[m].Host)
		}
	}
}

func processDomains(file io.Reader) {
	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		getMXRecord(scanner.Text())
	}
}

func main() {
	file, err := os.Open(filename)
	if err != nil {
		log.Fatal(err)
	}

	defer file.Close()
	processDomains(file)
}