human hand off fix

backend/app/services/mail_sender.py

@@ -237,6 +237,7 @@ async def deliver_email(
     message_id: str | None = None,
     inline_parts: list[ExtractedMimePart] | None = None,
     file_attachments: list[ExtractedMimePart] | None = None,
+    envelope_sender: str | None = None,
 ) -> str:
     _, sender_address = _sender_identity(sender, sender_name)
     message = build_message(
@@ -259,7 +260,7 @@ async def deliver_email(
     log_signed_mime_headers(signed)
     await send_via_smtp(
         signed,
-        sender=sender_address,
+        sender=envelope_sender or sender_address,
         recipients=[*recipients, *cc, *bcc],
     )
     return str(message["Message-ID"])