Checks X509 certificate subjects and SANs for given domain's MX servers

smtp_certificate_checker.go

package main

import (
	"crypto/tls"
	"crypto/x509"
	"fmt"
	"log"
	"net"
	"net/smtp"
	"os"
)

func printCertificateInfo(cert *x509.Certificate) {
	fmt.Printf(" Subject: %s\n", cert.Subject)
	fmt.Printf(" DNSNames:\n")
	for _, name := range cert.DNSNames {
		fmt.Printf("  %s\n", name)
	}
}

func main() {
	mxs, err := net.LookupMX(os.Args[1])
	if err != nil {
		log.Fatal(err)
	}

	for _, mx := range mxs {
		fmt.Printf("%s\n", mx.Host)
		client, err := smtp.Dial(mx.Host + ":25")
		if err != nil {
			log.Fatal(err)
		}

		err = client.StartTLS(&tls.Config{
			InsecureSkipVerify: true,
		})
		if err != nil {
			log.Fatal(err)
		}

		state, ok := client.TLSConnectionState()
		if !ok {
			log.Fatal("client.TLSConnectionState: !ok")
		}

		printCertificateInfo(state.PeerCertificates[0])
	}
}