diff --git a/src/daemon/mail_notify.py b/src/daemon/mail_notify.py
new file mode 100644
index 0000000..9e7d632
--- /dev/null
+++ b/src/daemon/mail_notify.py
@@ -0,0 +1,104 @@
+"""Mail 失败通知 — 以 system 身份通知发件人"""
+
+from __future__ import annotations
+
+import json
+import logging
+from datetime import datetime
+from pathlib import Path
+from typing import Optional
+
+from src.blackboard.models import Task
+from src.blackboard.operations import Blackboard
+
+logger = logging.getLogger(__name__)
+
+# 邮件通知正文模板（统一模板，包含所有可能的失败原因和建议）
+_NOTIFY_TEMPLATE = """你的邮件投递失败了。
+
+📧 原始邮件：「{title}」
+👤 收件人：{to_agent}
+❌ 失败原因：{reason}
+
+常见失败原因及处理建议：
+• no_reply_found：收件人未回复。建议重发邮件，或通过黑板任务方式联系
+• auth_failed：收件人认证失败。需检查 Agent 配置，联系姜维(jiangwei-infra)排查
+• crash_limit：收件人处理时多次崩溃。系统异常，建议稍后重试
+• task_timeout：处理超时。建议重发或通过其他方式联系
+• 其他原因：建议联系副军师(pangtong-fujunshi)排查
+
+——系统自动通知"""
+
+
+def _is_mail_project(db_path: Path) -> bool:
+    """从 db_path 推断是否为 _mail 项目"""
+    path_str = str(db_path)
+    return "/_mail/" in path_str or path_str.endswith("_mail.db")
+
+
+def notify_mail_failed(db_path: Path, original_mail_id: str,
+                       reason: str, detail: Optional[dict] = None) -> None:
+    """Mail 失败后以 system 身份给发件人发通知邮件
+
+    直接通过 Blackboard 创建 Task，不走 HTTP API。
+    防递归：检查原邮件 must_hives.system_notify，为 true 则跳过。
+    """
+    try:
+        bb = Blackboard(db_path)
+        original = bb.get_task(original_mail_id)
+        if not original:
+            logger.warning("notify_mail_failed: original mail %s not found", original_mail_id)
+            return
+
+        # 解析原邮件元数据
+        meta = json.loads(original.must_haves) if original.must_haves else {}
+
+        # 防递归：系统通知邮件失败不再发通知
+        if meta.get("system_notify"):
+            logger.info("Mail %s: system notify mail failed, skipping recursive notification", original_mail_id)
+            return
+
+        # 获取发件人（优先 assigned_by，fallback must_haves.from）
+        from_agent = original.assigned_by or meta.get("from", "")
+        to_agent = original.assignee or ""
+        title = original.title or ""
+
+        if not from_agent:
+            logger.warning("notify_mail_failed: cannot determine sender for mail %s", original_mail_id)
+            return
+
+        # 构造通知正文
+        text = _NOTIFY_TEMPLATE.format(
+            title=title,
+            to_agent=to_agent,
+            reason=reason,
+        )
+
+        # 创建通知邮件 Task
+        notify_id = f"mail-{int(datetime.now().timestamp() * 1000)}"
+        notify_meta = {
+            "type": "inform",
+            "performative": "inform",
+            "is_read": False,
+            "conversation_id": meta.get("conversation_id", ""),
+            "in_reply_to": original_mail_id,
+            "from": "system",
+            "system_notify": True,
+        }
+
+        notify_task = Task(
+            id=notify_id,
+            title=f"[投递失败] {title}",
+            description=text,
+            assignee=from_agent,
+            assigned_by="system",
+            must_haves=json.dumps(notify_meta, ensure_ascii=False),
+            task_type="mail",
+            status="pending",
+        )
+        bb.create_task(notify_task)
+        logger.info("Mail %s: sent failure notification to %s (reason=%s, notify_id=%s)",
+                     original_mail_id, from_agent, reason, notify_id)
+
+    except Exception as e:
+        logger.warning("notify_mail_failed: failed to send notification for mail %s: %s", original_mail_id, e)