handoff fixes

backend/app/services/mail_sender.py

@@ -1,5 +1,7 @@
 import email.policy
 import email.utils
+import logging
+from email.parser import BytesParser
 from email.message import EmailMessage, Message
 from email.mime.base import MIMEBase
 from email.mime.multipart import MIMEMultipart
@@ -13,6 +15,20 @@ from app.models import MailDomain
 from app.services.mime_body_extractor import ExtractedMimePart
 
 SMTP_POLICY = email.policy.SMTP.clone(max_line_length=998)
+logger = logging.getLogger(__name__)
+
+
+def log_outbound_mime_headers(message: Message) -> None:
+    logger.info(
+        "smtp_outbound_mime_headers message_id=%s in_reply_to=%s references=%s "
+        "from=%s to=%s subject=%s",
+        message.get("Message-ID"),
+        message.get("In-Reply-To"),
+        message.get("References"),
+        message.get("From"),
+        message.get("To"),
+        message.get("Subject"),
+    )
 
 
 def _mime_attachment(part: ExtractedMimePart) -> MIMEBase:
@@ -60,7 +76,9 @@ def build_message(
         root["Message-ID"] = generated_id
         if in_reply_to:
             root["In-Reply-To"] = in_reply_to
-        if references:
+            refs = references or [in_reply_to]
+            root["References"] = " ".join(refs)
+        elif references:
             root["References"] = " ".join(references)
 
         alternative = MIMEMultipart("alternative", policy=SMTP_POLICY)
@@ -94,7 +112,9 @@ def build_message(
     message["Message-ID"] = generated_id
     if in_reply_to:
         message["In-Reply-To"] = in_reply_to
-    if references:
+        refs = references or [in_reply_to]
+        message["References"] = " ".join(refs)
+    elif references:
         message["References"] = " ".join(references)
     message.set_content(body_text)
     if body_html:
@@ -115,6 +135,10 @@ def sign_message(message: Message, domain: MailDomain) -> bytes:
     ]
     if message.get("Cc"):
         signed_headers.insert(3, b"cc")
+    if message.get("In-Reply-To"):
+        signed_headers.append(b"in-reply-to")
+    if message.get("References"):
+        signed_headers.append(b"references")
     signature = dkim.sign(
         raw_message,
         domain.dkim_selector.encode(),
@@ -180,7 +204,9 @@ async def deliver_email(
         inline_parts=inline_parts,
         file_attachments=file_attachments,
     )
+    log_outbound_mime_headers(message)
     signed = sign_message(message, domain)
+    log_signed_mime_headers(signed)
     await send_via_smtp(
         signed,
         sender=sender,