@@ 0,0 1,83 @@
+From 5bea8e8c73511f90aa7c8b80fe49357862a84b92 Mon Sep 17 00:00:00 2001
+From: Drew DeVault <sir@cmpwn.com>
+Date: Mon, 6 Jan 2020 10:49:48 -0500
+Subject: [PATCH] email: generate message ID headers
+
+This GenerateMessageId is taken from go-message:
+
+https://github.com/emersion/go-message/blob/master/mail/mail.go
+
+I am the original author of this file; I agree to distribute it in
+alertmanager regardless of the original license.
+
+Signed-off-by: Drew DeVault <sir@cmpwn.com>
+---
+ notify/email/email.go | 25 +++++++++++++++++++++++++
+ 1 file changed, 25 insertions(+)
+
+diff --git a/notify/email/email.go b/notify/email/email.go
+index 73fcf577..529e4333 100644
+--- a/notify/email/email.go
++++ b/notify/email/email.go
+@@ -16,7 +16,9 @@ package email
+ import (
+ 	"bytes"
+ 	"context"
++	"crypto/rand"
+ 	"crypto/tls"
++	"encoding/binary"
+ 	"fmt"
+ 	"mime"
+ 	"mime/multipart"
+@@ -25,11 +27,13 @@ import (
+ 	"net/mail"
+ 	"net/smtp"
+ 	"net/textproto"
++	"os"
+ 	"strings"
+ 	"time"
+ 
+ 	"github.com/go-kit/kit/log"
+ 	"github.com/go-kit/kit/log/level"
++	"github.com/martinlindhe/base36"
+ 	"github.com/pkg/errors"
+ 	commoncfg "github.com/prometheus/common/config"
+ 
+@@ -241,6 +245,7 @@ func (n *Email) Notify(ctx context.Context, as ...*types.Alert) (bool, error) {
+ 
+ 	fmt.Fprintf(buffer, "Date: %s\r\n", time.Now().Format(time.RFC1123Z))
+ 	fmt.Fprintf(buffer, "Content-Type: multipart/alternative;  boundary=%s\r\n", multipartWriter.Boundary())
++	fmt.Fprintf(buffer, "Message-ID: %s\r\n", generateMessageID())
+ 	fmt.Fprintf(buffer, "MIME-Version: 1.0\r\n\r\n")
+ 
+ 	// TODO: Add some useful headers here, such as URL of the alertmanager
+@@ -314,6 +319,26 @@ func (n *Email) Notify(ctx context.Context, as ...*types.Alert) (bool, error) {
+ 	return false, nil
+ }
+ 
++// Generates an RFC 2822-compliant Message-Id based on the informational draft
++// "Recommendations for generating Message IDs", for lack of a better
++// authoritative source.
++func generateMessageID() string {
++	var (
++		now   bytes.Buffer
++		nonce []byte = make([]byte, 8)
++	)
++	binary.Write(&now, binary.BigEndian, time.Now().UnixNano())
++	rand.Read(nonce)
++	hostname, err := os.Hostname()
++	if err != nil {
++		hostname = "localhost"
++	}
++	return fmt.Sprintf("<%s.%s@%s>",
++		base36.EncodeBytes(now.Bytes()),
++		base36.EncodeBytes(nonce),
++		hostname)
++}
++
+ type loginAuth struct {
+ 	username, password string
+ }
+-- 
+2.24.1
+