diff --git a/pkg/mail/domain.go b/pkg/mail/domain.go
new file mode 100644
index 000000000..f3c65bec8
--- /dev/null
+++ b/pkg/mail/domain.go
@@ -0,0 +1,36 @@
+// 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 (
+	"net/url"
+
+	"code.vikunja.io/api/pkg/config"
+)
+
+// GetMailDomain returns the hostname from the configured public URL,
+// or "vikunja" as a fallback. Used for RFC 5322 compliant Message-ID
+// and thread ID generation.
+func GetMailDomain() string {
+	publicURL := config.ServicePublicURL.GetString()
+	if publicURL != "" {
+		if parsedURL, err := url.Parse(publicURL); err == nil && parsedURL.Hostname() != "" {
+			return parsedURL.Hostname()
+		}
+	}
+	return "vikunja"
+}
diff --git a/pkg/mail/domain_test.go b/pkg/mail/domain_test.go
new file mode 100644
index 000000000..9055090d2
--- /dev/null
+++ b/pkg/mail/domain_test.go
@@ -0,0 +1,46 @@
+// 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 TestGetMailDomain(t *testing.T) {
+	t.Run("returns fallback when public URL is empty", func(t *testing.T) {
+		config.ServicePublicURL.Set("")
+		assert.Equal(t, "vikunja", GetMailDomain())
+	})
+
+	t.Run("extracts hostname from public URL", func(t *testing.T) {
+		config.ServicePublicURL.Set("https://tasks.example.com/")
+		assert.Equal(t, "tasks.example.com", GetMailDomain())
+	})
+
+	t.Run("extracts hostname without port", func(t *testing.T) {
+		config.ServicePublicURL.Set("https://tasks.example.com:8080/")
+		assert.Equal(t, "tasks.example.com", GetMailDomain())
+	})
+
+	t.Run("returns fallback for invalid URL", func(t *testing.T) {
+		config.ServicePublicURL.Set("://bad")
+		assert.Equal(t, "vikunja", GetMailDomain())
+	})
+}