diff --git a/shortcuts/mail/emlbuilder/builder.go b/shortcuts/mail/emlbuilder/builder.go
index d4b5b22f9..5c0ba60bb 100644
--- a/shortcuts/mail/emlbuilder/builder.go
+++ b/shortcuts/mail/emlbuilder/builder.go
@@ -86,6 +86,7 @@ type Builder struct {
 	inReplyTo                 string // raw value, without angle brackets
 	references                string // space-separated list of message IDs, with angle brackets
 	lmsReplyToMessageID       string // Lark internal message_id of the original message
+	lmsReplyType              string // REPLY or FORWARD, written as X-LMS-Reply-Type header
 	textBody                  []byte
 	htmlBody                  []byte
 	calendarBody              []byte
@@ -391,6 +392,26 @@ func (b Builder) LMSReplyToMessageID(id string) Builder {
 	return b
 }
 
+// LMSReplyType sets the reply type so the backend can label the original message
+// as replied ("REPLY") or forwarded ("FORWARD"). Written as the X-LMS-Reply-Type
+// header. Only "REPLY" and "FORWARD" are accepted; any other value is a no-op
+// (no header emitted) to avoid leaking unexpected values.
+// Returns an error builder if the value contains CR or LF.
+func (b Builder) LMSReplyType(t string) Builder {
+	if b.err != nil {
+		return b
+	}
+	if t != "REPLY" && t != "FORWARD" {
+		return b
+	}
+	if err := validateHeaderValue(t); err != nil {
+		b.err = err
+		return b
+	}
+	b.lmsReplyType = t
+	return b
+}
+
 // References sets the References header value verbatim.
 // Typically a space-separated list of message IDs including angle brackets,
 // e.g. "<id1@host> <id2@host>".
@@ -721,6 +742,9 @@ func (b Builder) Build() ([]byte, error) {
 		if b.lmsReplyToMessageID != "" {
 			writeHeader(&buf, "X-LMS-Reply-To-Message-Id", b.lmsReplyToMessageID)
 		}
+		if b.lmsReplyType != "" {
+			writeHeader(&buf, "X-LMS-Reply-Type", b.lmsReplyType)
+		}
 	}
 	if b.references != "" {
 		writeHeader(&buf, "References", b.references)
diff --git a/shortcuts/mail/emlbuilder/builder_test.go b/shortcuts/mail/emlbuilder/builder_test.go
index 024b602c0..c78fec9da 100644
--- a/shortcuts/mail/emlbuilder/builder_test.go
+++ b/shortcuts/mail/emlbuilder/builder_test.go
@@ -264,6 +264,55 @@ func TestBuild_LMSReplyToMessageID(t *testing.T) {
 	}
 }
 
+// TestBuild_LMSReplyType verifies build LMS reply type header.
+func TestBuild_LMSReplyType(t *testing.T) {
+	raw, err := New().
+		From("", "alice@example.com").
+		To("", "bob@example.com").
+		Subject("Re: hello").
+		Date(fixedDate).
+		InReplyTo("original@smtp").
+		LMSReplyToMessageID("740000000000000067").
+		LMSReplyType("REPLY").
+		TextBody([]byte("my reply")).
+		Build()
+	if err != nil {
+		t.Fatal(err)
+	}
+	eml := string(raw)
+
+	got := headerValue(eml, "X-LMS-Reply-Type")
+	if got != "REPLY" {
+		t.Errorf("X-LMS-Reply-Type: got %q, want REPLY", got)
+	}
+	if !strings.Contains(eml, "X-LMS-Reply-Type: REPLY") {
+		t.Errorf("eml should contain header line \"X-LMS-Reply-Type: REPLY\", got:\n%s", eml)
+	}
+}
+
+// TestBuild_LMSReplyType_InvalidValueNotWritten verifies an invalid reply type is ignored.
+func TestBuild_LMSReplyType_InvalidValueNotWritten(t *testing.T) {
+	raw, err := New().
+		From("", "alice@example.com").
+		To("", "bob@example.com").
+		Subject("Re: hello").
+		Date(fixedDate).
+		InReplyTo("original@smtp").
+		LMSReplyToMessageID("740000000000000067").
+		LMSReplyType("BOGUS").
+		TextBody([]byte("my reply")).
+		Build()
+	if err != nil {
+		t.Fatal(err)
+	}
+	eml := string(raw)
+
+	got := headerValue(eml, "X-LMS-Reply-Type")
+	if got != "" {
+		t.Errorf("X-LMS-Reply-Type should be absent for invalid value, got %q", got)
+	}
+}
+
 // TestBuild_LMSReplyToMessageID_NotWrittenWithoutInReplyTo verifies build LMS reply to message ID not written without in reply to.
 func TestBuild_LMSReplyToMessageID_NotWrittenWithoutInReplyTo(t *testing.T) {
 	raw, err := New().
diff --git a/shortcuts/mail/mail_forward.go b/shortcuts/mail/mail_forward.go
index 466db2e12..4c15b9dcf 100644
--- a/shortcuts/mail/mail_forward.go
+++ b/shortcuts/mail/mail_forward.go
@@ -229,6 +229,7 @@ var MailForward = common.Shortcut{
 		}
 		if messageId != "" {
 			bld = bld.LMSReplyToMessageID(messageId)
+			bld = bld.LMSReplyType("FORWARD")
 		}
 		useHTML := !plainText && (bodyIsHTML(body) || bodyIsHTML(orig.bodyRaw) || sigResult != nil)
 		if strings.TrimSpace(inlineFlag) != "" && !useHTML {
diff --git a/shortcuts/mail/mail_reply.go b/shortcuts/mail/mail_reply.go
index a7674295e..ddbf2923c 100644
--- a/shortcuts/mail/mail_reply.go
+++ b/shortcuts/mail/mail_reply.go
@@ -239,6 +239,7 @@ var MailReply = common.Shortcut{
 		}
 		if messageId != "" {
 			bld = bld.LMSReplyToMessageID(messageId)
+			bld = bld.LMSReplyType("REPLY")
 		}
 		var autoResolvedPaths []string
 		var composedHTMLBody string
diff --git a/shortcuts/mail/mail_reply_all.go b/shortcuts/mail/mail_reply_all.go
index e7eaf02ce..78c82d89b 100644
--- a/shortcuts/mail/mail_reply_all.go
+++ b/shortcuts/mail/mail_reply_all.go
@@ -248,6 +248,7 @@ var MailReplyAll = common.Shortcut{
 		}
 		if messageId != "" {
 			bld = bld.LMSReplyToMessageID(messageId)
+			bld = bld.LMSReplyType("REPLY")
 		}
 		var autoResolvedPaths []string
 		var composedHTMLBody string