diff --git a/docs/superpowers/plans/2026-08-25-mail.md b/docs/superpowers/plans/2026-08-25-mail.md
index 235adbf..3e6ada0 100644
--- a/docs/superpowers/plans/2026-08-25-mail.md
+++ b/docs/superpowers/plans/2026-08-25-mail.md
@@ -5473,9 +5473,10 @@ function fakeSend(result: Partial<MailSendResult> = {}): { send: (input: MailSen
 }
 
 describe('toInboundMail', () => {
-  it('线程取 References 末项(没 In-Reply-To);只带存下来的附件;纯 html 剥成正文', () => {
+  it('线程取 References **首项**(即这一串的根);只带存下来的附件;纯 html 剥成正文', () => {
     const mail: InboundMail = toInboundMail(detail);
-    expect(mail.threadId).toBe('<q1@we.com>');
+    // 根 = 我方出站登记的那个 threadId —— 取最近一跳会让①级归属跑一个来回就漂
+    expect(mail.threadId).toBe('<root@we.com>');
     expect(mail.from).toBe('buyer@acme.com');
     expect(mail.to).toEqual(['sales@we.com']);
     expect(mail.cc).toEqual(['ops@we.com']);
@@ -5707,9 +5708,18 @@ import {
 } from '@modules/mail';
 import type { ToolHandler } from '@modules/toolbox';
 
-/** mail 的入站详情 → channel 的 InboundMail:只带存下来的附件;线程 = In-Reply-To ⊃ References 末项 */
+/**
+ * mail 的入站详情 → channel 的 InboundMail:只带存下来的附件。
+ *
+ * **线程取的是根,不是最近一跳**:我方出站登记的 threadId 恒是这一串的**首封**
+ * (`channelTransport` 回的是 `input.threadId ?? messageId`,channel 据此 `recordOutbound`),
+ * 而归属解析①级是精确等值比对。若这里取 `inReplyTo`(最近一跳),跑一个来回就漂:
+ * 我方 M1 → 客户回 M1(命中)→ 我方再回(实际 M2、仍记在 M1 名下)→ 客户回 M2 →
+ * 取到 M2,而库里从没记过 M2 ⇒ ①级落空,退到③地址+时间窗。
+ * `references[0]` 才稳定:客户端续链时首项恒是首封;客户端只给 In-Reply-To 时退回它。
+ */
 export function toInboundMail(mail: MailInboundDetail): InboundMail {
-  const threadId = mail.inReplyTo ?? mail.references.at(-1);
+  const threadId = mail.references[0] ?? mail.inReplyTo ?? undefined;
   return {
     messageId: mail.messageId,
     ...(threadId === undefined ? {} : { threadId }),
@@ -5767,7 +5777,8 @@ function attachmentsOf(value: unknown): MailSendInput['attachments'] {
     if (typeof v !== 'object' || v === null) return [];
     const a = v as Record<string, unknown>;
     const uploadRef = typeof a['uploadRef'] === 'string' && a['uploadRef'] !== '' ? a['uploadRef'] : undefined;
-    if (uploadRef === undefined) return [];
+    // 没给 uploadRef 的附件**不静默丢掉**:活动说「带上 q.pdf」,信发了附件没了还回 queued:true,是界面撒谎
+    if (uploadRef === undefined) throw new MailSendInvalidError([`附件 ${String(a['name'] ?? '(无名)')} 没给 uploadRef`]);
     const name = typeof a['name'] === 'string' && a['name'] !== '' ? a['name'] : uploadRef;
     return [{ name, contentType: 'application/octet-stream', uploadRef }];
   });