email reply fixes

backend/app/services/mail_sender.py

@@ -31,6 +31,21 @@ def log_outbound_mime_headers(message: Message) -> None:
     )
 
 
+def log_signed_mime_headers(message_bytes: bytes) -> None:
+    """Log threading headers from the final wire-format MIME (post-DKIM)."""
+    parsed = BytesParser(policy=SMTP_POLICY).parsebytes(message_bytes)
+    logger.info(
+        "smtp_signed_mime_headers message_id=%s in_reply_to=%s references=%s "
+        "from=%s to=%s subject=%s",
+        parsed.get("Message-ID"),
+        parsed.get("In-Reply-To"),
+        parsed.get("References"),
+        parsed.get("From"),
+        parsed.get("To"),
+        parsed.get("Subject"),
+    )
+
+
 def _mime_attachment(part: ExtractedMimePart) -> MIMEBase:
     maintype, _, subtype = part.content_type.partition("/")
     if not subtype: