From 754922b2503ca8569df7c3c4de7d1709c01c413b Mon Sep 17 00:00:00 2001
From: William Petit <wpetit@cadoles.com>
Date: Thu, 9 Jul 2020 14:00:17 +0200
Subject: [PATCH] Generate pseudo-random Message-Id header for each outgoing
 emails

---
 internal/mail/send.go | 47 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/internal/mail/send.go b/internal/mail/send.go
index 05da094..38a00eb 100644
--- a/internal/mail/send.go
+++ b/internal/mail/send.go
@@ -2,11 +2,21 @@ package mail
 
 import (
 	"crypto/tls"
+	"fmt"
+	"math/rand"
+	"net/mail"
+	"strconv"
+	"strings"
+	"time"
 
 	"github.com/pkg/errors"
 	gomail "gopkg.in/mail.v2"
 )
 
+var (
+	ErrUnexpectedEmailAddressFormat = errors.New("unexpected email address format")
+)
+
 type SendFunc func(*SendOption)
 
 type SendOption struct {
@@ -124,6 +134,20 @@ func (m *Mailer) Send(funcs ...SendFunc) error {
 		message.SetHeader(h.Field, h.Values...)
 	}
 
+	froms := message.GetHeader("From")
+
+	var sendDomain string
+
+	if len(froms) > 0 {
+		sendDomain, err = extractEmailDomain(froms[0])
+		if err != nil {
+			return err
+		}
+	}
+
+	messageID := generateMessageID(sendDomain)
+	message.SetHeader("Message-Id", messageID)
+
 	message.SetBody(opt.Body.Type, opt.Body.Content, opt.Body.PartSetting)
 
 	for _, b := range opt.AlternativeBodies {
@@ -158,3 +182,26 @@ func (m *Mailer) openConnection() (gomail.SendCloser, error) {
 
 	return conn, nil
 }
+
+func extractEmailDomain(email string) (string, error) {
+	address, err := mail.ParseAddress(email)
+	if err != nil {
+		return "", errors.Wrapf(err, "could not parse email address '%s'", email)
+	}
+
+	addressParts := strings.SplitN(address.Address, "@", 2)
+	if len(addressParts) != 2 { // nolint: gomnd
+		return "", errors.WithStack(ErrUnexpectedEmailAddressFormat)
+	}
+
+	domain := addressParts[1]
+
+	return domain, nil
+}
+
+func generateMessageID(domain string) string {
+	// Based on https://www.jwz.org/doc/mid.html
+	timestamp := strconv.FormatInt(time.Now().UnixNano(), 36)
+	random := strconv.FormatInt(rand.Int63(), 36)
+	return fmt.Sprintf("<%s.%s@%s>", timestamp, random, domain)
+}