fix(mail): set RFC 5322 compliant Message-ID using public URL domain

Fixes #2522

pkg/mail/send_mail.go

@@ -17,7 +17,9 @@
 package mail
 
 import (
+	"crypto/rand"
 	"embed"
+	"encoding/hex"
 	"fmt"
 	"io"
 
@@ -78,6 +80,13 @@ func SendTestMail(opts *Opts) error {
 func getMessage(opts *Opts) *mail.Msg {
 	m := mail.NewMsg()
 	m.SetUserAgent("Vikunja " + version.Version)
+
+	// Set an RFC 5322 compliant Message-ID using the public URL domain
+	// instead of relying on os.Hostname() which is unreliable in containers.
+	randBytes := make([]byte, 16)
+	_, _ = rand.Read(randBytes)
+	messageID := hex.EncodeToString(randBytes) + "@" + GetMailDomain()
+	m.SetMessageIDWithValue(messageID)
 	if opts.From == "" {
 		opts.From = "Vikunja <" + config.MailerFromEmail.GetString() + ">"
 	}

pkg/mail/send_mail_test.go

@@ -0,0 +1,42 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package mail
+
+import (
+	"testing"
+
+	"code.vikunja.io/api/pkg/config"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetMessageSetsMessageID(t *testing.T) {
+	config.ServicePublicURL.Set("https://tasks.example.com/")
+	config.MailerFromEmail.Set("test@example.com")
+
+	opts := &Opts{
+		To:          "recipient@example.com",
+		Subject:     "Test",
+		Message:     "Hello",
+		ContentType: ContentTypePlain,
+	}
+
+	m := getMessage(opts)
+
+	msgID := m.GetMessageID()
+	assert.NotEmpty(t, msgID)
+	assert.Contains(t, msgID, "@tasks.example.com>")
+}