26:def build_mime_message(
27-    from_email: str,
28-    to_email: str,
29-    subject: str,
30-    html_body: str = "",
31-    text_body: str = "",
32-    attachments: Optional[list] = None,   # list of {filename, content(bytes), mime_type}
33-    unsubscribe_url: str = "",
34-    unsubscribe_mailto: str = "",
35-) -> MIMEMultipart:
36-    """Build a real MIME multipart message. From header always equals the authenticated account."""
37-    attachments = attachments or []
38-    # RFC 2046 structure: when attachments exist, build
39-    # multipart/mixed[ alternative/plain+html, attachment... ]; attachments are
40-    # NEVER siblings of the body alternatives (multipart/alternative only holds
41-    # the same content in different formats).
42-    body = MIMEMultipart("alternative") if (html_body and text_body) else None
43-    msg = MIMEMultipart("mixed") if attachments else (
44-        MIMEMultipart("alternative") if (html_body and text_body) else MIMEMultipart("mixed"))
45-    msg["From"] = from_email  # MUST be the authenticated Gmail account (never spoofed)
46-    msg["To"] = to_email
47-    msg["Subject"] = subject
48-    msg["Date"] = formatdate(localtime=True)
49-    msg["Message-ID"] = make_msgid()
50-    msg["X-Mailer"] = "BulkGmailMailer (permission-based sending)"
51-
52-    # List-Unsubscribe headers for compliance
53-    if unsubscribe_url:
54-        parts = [f"<{unsubscribe_url}>"]
55-        if unsubscribe_mailto:
56-            parts.append(f"<mailto:{unsubscribe_mailto}>")
57-        msg["List-Unsubscribe"] = ", ".join(parts)
58-        msg["List-Unsubscribe-Post"] = "List-Unsubscribe=One-Click"
59-
60-    # Text part first, then HTML (preferred last in alternative)
61-    if text_body:
62-        (body or msg).attach(MIMEText(text_body, "plain", "utf-8"))
63-    if html_body:
64-        (body or msg).attach(MIMEText(html_body, "html", "utf-8"))
65-    if body is not None:
66-        msg.attach(body)
67-
68-    # Attachments
69-    for att in attachments:
70-        data = att["content"]
71-        ctype = att.get("mime_type") or "application/octet-stream"