diff --git a/apps/api/src/app/__tests__/mail-wiring.test.ts b/apps/api/src/app/__tests__/mail-wiring.test.ts
index 1444d3f..9459d66 100644
--- a/apps/api/src/app/__tests__/mail-wiring.test.ts
+++ b/apps/api/src/app/__tests__/mail-wiring.test.ts
@@ -41,9 +41,11 @@ 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>');
+    // 不是末项 <q1@we.com>:我方出站登记的恒是首封,①级归属是精确等值比对 ——
+    // 取最近一跳的话,跑一个来回(我方再回一封)线程就漂到库里没记过的 Message-ID 上,①级落空退③级
+    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']);
@@ -87,12 +89,23 @@ describe('mailSendTool', () => {
   const base = { connection: { account: 'default' }, operation: undefined, idempotencyKey: 'k-1' };
   it('数组 / 逗号串都认作收件人;body → text;幂等键与连接配置的账号带过去;出参保留 queued', async () => {
     const mail = fakeSend();
-    const result = await mailSendTool(mail)({ ...base, connection: { account: 'acc-2' }, input: { to: 'a@x.com; b@x.com', subject: 's', body: '正文', inReplyTo: '<m1@a>', attachments: [{ uploadRef: 'up-1', name: 'q.pdf' }, { name: 'no-ref' }] } });
+    const result = await mailSendTool(mail)({ ...base, connection: { account: 'acc-2' }, input: { to: 'a@x.com; b@x.com', subject: 's', body: '正文', inReplyTo: '<m1@a>', attachments: [{ uploadRef: 'up-1', name: 'q.pdf' }] } });
     expect(result).toEqual({ ok: true, output: { queued: true, outboundId: 'out-1', messageId: '<out-1@we.com>', state: 'sent' } });
     expect(mail.calls[0]).toMatchObject({ accountId: 'acc-2', to: ['a@x.com', 'b@x.com'], text: '正文', inReplyTo: '<m1@a>', references: ['<m1@a>'], idempotencyKey: 'k-1', attachments: [{ uploadRef: 'up-1', name: 'q.pdf' }], requestedBy: { kind: 'tool', tool: 'mail.send' } });
     await mailSendTool(mail)({ ...base, input: { to: ['a@x.com'], subject: 's', body: 'b' } });
     expect(mail.calls[1]?.accountId).toBeUndefined();
   });
+  it('没给 uploadRef 的附件不静默丢掉:整次调用判 invalid,一封都不发', async () => {
+    // 模块那一层本就是硬的(既没字节也没 ref 就抛 MailSendInvalidError);app 层不许比它松 ——
+    // 悄悄丢掉附件再回 queued:true,等于让界面撒谎
+    const mail = fakeSend();
+    const result = await mailSendTool(mail)({ ...base, input: { to: ['a@x.com'], subject: 's', body: 'b', attachments: [{ uploadRef: 'up-1', name: 'q.pdf' }, { name: 'no-ref' }] } });
+    expect(result.ok).toBe(false);
+    expect(result.error?.code).toBe('invalid');
+    expect(result.error?.retryable).toBe(false);
+    expect(result.error?.message).toContain('no-ref');
+    expect(mail.calls).toHaveLength(0);
+  });
   it('dryRun 只校验不发', async () => {
     const mail = fakeSend();
     expect(await mailSendTool(mail)({ ...base, input: { to: ['x@y.com'], subject: '询价', body: 'b', dryRun: true } })).toEqual({ ok: true, output: { queued: false, dryRun: true } });
diff --git a/apps/api/src/app/mail-wiring.ts b/apps/api/src/app/mail-wiring.ts
index c135221..8dc3772 100644
--- a/apps/api/src/app/mail-wiring.ts
+++ b/apps/api/src/app/mail-wiring.ts
@@ -18,9 +18,18 @@ 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 }),
@@ -80,7 +89,8 @@ 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 }];
   });