fix(GODT-2576): Correctly handle Forwarded messages from Thunderbird

Thunderbird uses `In-Reply-To` with `X-Forwarded-Message-Id` to signal
to the SMTP server that it is forwarding a message.

pkg/message/parser.go

@@ -60,6 +60,7 @@ type Message struct {
 	References []string
 	ExternalID string
 	InReplyTo  string
+	XForward   string
 }
 
 type Attachment struct {
@@ -520,6 +521,9 @@ func parseMessageHeader(h message.Header, allowInvalidAddressLists bool) (Messag
 		case "in-reply-to":
 			m.InReplyTo = regexp.MustCompile("<(.*)>").ReplaceAllString(fields.Value(), "$1")
 
+		case "x-forwarded-message-id":
+			m.XForward = regexp.MustCompile("<(.*)>").ReplaceAllString(fields.Value(), "$1")
+
 		case "references":
 			for _, ref := range strings.Fields(fields.Value()) {
 				for _, ref := range strings.Split(ref, ",") {

pkg/message/parser_test.go

@@ -786,6 +786,16 @@ func TestParseTextPlainWithDocxAttachmentCyrillic(t *testing.T) {
 	assert.Equal(t, "АБВГДЃЕЖЗЅИЈКЛЉМНЊОПРСТЌУФХЧЏЗШ.docx", m.Attachments[0].Name)
 }
 
+func TestParseInReplyToAndXForward(t *testing.T) {
+	f := getFileReader("text_plain_utf8_reply_to_and_x_forward.eml")
+
+	m, err := Parse(f)
+	require.NoError(t, err)
+
+	require.Equal(t, "00000@protonmail.com", m.XForward)
+	require.Equal(t, "00000@protonmail.com", m.InReplyTo)
+}
+
 func TestPatchNewLineWithHtmlBreaks(t *testing.T) {
 	{
 		input := []byte("\nfoo\nbar\n\n\nzz\nddd")

pkg/message/testdata/text_plain_utf8_reply_to_and_x_forward.eml

@@ -0,0 +1,7 @@
+From: Sender <sender@pm.me>
+To: Receiver <receiver@pm.me>
+Content-Type: text/plain; charset=utf-8
+X-Forwarded-Message-Id: <00000@protonmail.com>
+In-Reply-To: <00000@protonmail.com>
+
+body