From 34b26a792bcfa79f3efe07fc80b95a0da4e87f36 Mon Sep 17 00:00:00 2001
From: Pierre-Olivier Mercier <nemunaire@nemunai.re>
Date: Wed, 22 Oct 2025 12:27:22 +0700
Subject: [PATCH] Prepend Return-Path header when receiving email

---
 internal/lmtp/server.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/internal/lmtp/server.go b/internal/lmtp/server.go
index 1d9a720..a9b36b9 100644
--- a/internal/lmtp/server.go
+++ b/internal/lmtp/server.go
@@ -92,6 +92,10 @@ func (s *Session) Data(r io.Reader) error {
 
 	log.Printf("LMTP: Received %d bytes", len(emailData))
 
+	// Prepend Return-Path header from envelope sender
+	returnPath := fmt.Sprintf("Return-Path: <%s>\r\n", s.from)
+	emailData = append([]byte(returnPath), emailData...)
+
 	// Process email for each recipient
 	// LMTP requires per-recipient status, but go-smtp handles this internally
 	for _, recipient := range s.recipients {