Include `Message-ID` and `Date` in outbound emails

Closes: #82

monitor/mailutils.go

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Opsmate, Inc.
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License, v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+//
+// This software is distributed WITHOUT A WARRANTY OF ANY KIND.
+// See the Mozilla Public License for details.
+
+package monitor
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+)
+
+const mailDateFormat = "Mon, 2 Jan 2006 15:04:05 -0700"
+
+func generateMessageID() string {
+	var randomBytes [16]byte
+	if _, err := rand.Read(randomBytes[:]); err != nil {
+		panic(err)
+	}
+	return hex.EncodeToString(randomBytes[:]) + "@selfhosted.certspotter.org"
+}

monitor/notify.go

@@ -20,6 +20,7 @@ import (
 	"path/filepath"
 	"strings"
 	"sync"
+	"time"
 )
 
 var stdoutMu sync.Mutex
@@ -73,6 +74,8 @@ func sendEmail(ctx context.Context, to []string, notif notification) error {
 
 	fmt.Fprintf(stdin, "To: %s\n", strings.Join(to, ", "))
 	fmt.Fprintf(stdin, "Subject: [certspotter] %s\n", notif.Summary())
+	fmt.Fprintf(stdin, "Date: %s\n", time.Now().Format(mailDateFormat))
+	fmt.Fprintf(stdin, "Message-ID: <%s>\n", generateMessageID())
 	fmt.Fprintf(stdin, "Mime-Version: 1.0\n")
 	fmt.Fprintf(stdin, "Content-Type: text/plain; charset=US-ASCII\n")
 	fmt.Fprintf(stdin, "X-Mailer: certspotter\n")