diff --git a/decnet/orchestrator/drivers/smtp_relay.py b/decnet/orchestrator/drivers/smtp_relay.py
index 6e8bf12f..4c1be0df 100644
--- a/decnet/orchestrator/drivers/smtp_relay.py
+++ b/decnet/orchestrator/drivers/smtp_relay.py
@@ -8,6 +8,7 @@ Called by the realism worker's smtp probe listener, not the main tick loop.
 """
 from __future__ import annotations
 
+import email
 import smtplib
 from pathlib import Path
 from typing import Any
@@ -15,6 +16,19 @@ from typing import Any
 _ARTIFACTS_ROOT_DEFAULT = "/var/lib/decnet/artifacts"
 
 
+def _ensure_from_header(body: bytes, mail_from: str) -> bytes:
+    """Return body with a From: header added if one is absent."""
+    try:
+        msg = email.message_from_bytes(body)
+    except Exception:
+        return body
+    if msg["From"]:
+        return body
+    # Prepend the header before the existing content.
+    header_line = f"From: {mail_from}\r\n".encode()
+    return header_line + body
+
+
 def forward_probe(
     *,
     svc_cfg: dict[str, Any],
@@ -47,6 +61,11 @@ def forward_probe(
     upstream_pass  = (svc_cfg.get("upstream_pass") or "").strip()
     envelope_from  = (svc_cfg.get("upstream_sender") or "").strip() or mail_from
 
+    # Ensure the message has a From: header so mail clients show the attacker's
+    # address rather than falling back to the envelope sender (upstream_sender).
+    # Minimal relay-test scripts often omit headers entirely.
+    body = _ensure_from_header(body, mail_from)
+
     try:
         with smtplib.SMTP(upstream_host, upstream_port, timeout=15) as conn:
             conn.ehlo()