diff --git a/components/email/email-composer.tsx b/components/email/email-composer.tsx
index b9d0e9bcf..e34180cb6 100644
--- a/components/email/email-composer.tsx
+++ b/components/email/email-composer.tsx
@@ -46,7 +46,7 @@ import { appendPlainTextSignature, getPlainTextSignature, plainTextBodyHasSignat
 import { findComposeIdentityId, findDraftIdentityId, findReplyIdentityId, resolveReplyFrom } from "@/lib/reply-identity";
 import { buildReplyRecipients, isSelfSent } from "@/lib/reply-recipients";
 import { computeReplyThreadingHeaders, type ReplyThreadingHeaders } from "@/lib/email-threading";
-import { RecipientsRejectedError, RequestTimeoutError, ScheduleTooLateError, formatRejectedRecipients } from "@/lib/jmap/client";
+import { RecipientsRejectedError, RequestTimeoutError, ScheduleTooLateError, SendUnconfirmedError, formatRejectedRecipients } from "@/lib/jmap/client";
 import {
   rewriteCidImagesForEditor,
   replaceInlineImagePlaceholders,
@@ -2607,8 +2607,9 @@ export function EmailComposer({
       // A timeout is not a clean failure: the submission may have reached the
       // server and gone out, with only the answer lost. Saying "send failed"
       // would invite a re-send and a duplicate, so point at Sent instead (#702).
+      // The same goes for a send the server answered without confirming it.
       toast.error(
-        err instanceof RequestTimeoutError
+        err instanceof RequestTimeoutError || err instanceof SendUnconfirmedError
           ? t('send_timeout')
           : err instanceof ScheduleTooLateError
             ? t('schedule_send_too_late')
diff --git a/lib/__tests__/jmap-draft-lifecycle.test.ts b/lib/__tests__/jmap-draft-lifecycle.test.ts
index f876368a0..0b657dfc1 100644
--- a/lib/__tests__/jmap-draft-lifecycle.test.ts
+++ b/lib/__tests__/jmap-draft-lifecycle.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest';
-import { JMAPClient } from '../jmap/client';
+import { JMAPClient, SendUnconfirmedError } from '../jmap/client';
 
 /**
  * #849: replacing a draft (autosave, send) used to bundle the destroy of the
@@ -238,7 +238,10 @@ describe('compose failure recovery', () => {
   beforeEach(() => { vi.restoreAllMocks(); vi.spyOn(console, 'error').mockImplementation(() => {}); });
   it.each([{ methodError: true }, { missingSubmission: true }])('does not claim success or delete the old draft without confirmation: %j', async options => {
     const client = createClient(); const captured = mockFlow(options);
-    await expect(client.sendEmail(['bob@example.com'], 'Subject', 'body', undefined, undefined, 'identity-1', 'user@example.com', 'old-draft')).rejects.toThrow();
+    await expect(client.sendEmail(['bob@example.com'], 'Subject', 'body', undefined, undefined, 'identity-1', 'user@example.com', 'old-draft')).rejects.toThrow(
+      // Only the missing submission is unconfirmed; a method error is a plain failure.
+      options.missingSubmission ? SendUnconfirmedError : Error,
+    );
     expect(emailSetCalls(captured).some(call => call.destroy)).toBe(false);
   });
   it('accepts empty notCreated maps as success', async () => {
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index f7a64de5c..320187428 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -775,6 +775,17 @@ export class RecipientsRejectedError extends Error {
   }
 }
 
+/**
+ * The send request came back without an EmailSubmission: nothing confirms the
+ * message left, and it may still have. The draft is kept.
+ */
+export class SendUnconfirmedError extends Error {
+  constructor() {
+    super('Send confirmation was not received. Check Sent before sending again. Your draft has been kept.');
+    this.name = 'SendUnconfirmedError';
+  }
+}
+
 /** "report.pdf" -> "report (2).pdf", the way Stalwart's onExists "rename" names copies. */
 function numberedFileName(name: string, n: number): string {
   const dot = name.lastIndexOf('.');
@@ -4193,7 +4204,7 @@ export class JMAPClient implements IJMAPClient {
     // all. Nothing confirms the message left: report a failure and keep the
     // old draft rather than replacing it as if the send had worked.
     if (!emailSubmissionId) {
-      throw new Error('Send confirmation was not received. Check Sent before sending again. Your draft has been kept.');
+      throw new SendUnconfirmedError();
     }
 
     // With every recipient refused nothing was queued, yet the submission