fix: stop reporting a send as failed when only filing it into Sent failed

`sendEmail` threw on any method error in the response. Stalwart appends
one for the implicit `onSuccessUpdateEmail` Email/set that moves the
sent message from Drafts to Sent, so a message that had already gone
out showed "Send failed" and a retry sent it a second time. A method
error that follows a created submission is now returned as a filing
warning, like the record-level filing failures already were; errors
before the submission exists still fail the send.

src/api/__tests__/email-submission.test.ts

@@ -11,7 +11,7 @@ vi.mock('../jmap-client', () => ({
 }));
 
 import { jmapClient } from '../jmap-client';
-import { rescheduleScheduledSend } from '../email';
+import { rescheduleScheduledSend, sendEmail } from '../email';
 
 const mockRequest = jmapClient.request as ReturnType<typeof vi.fn>;
 
@@ -102,3 +102,57 @@ describe('rescheduleScheduledSend', () => {
     expect(mockRequest.mock.calls[0][0][0][1].create.replacement).toEqual({ emailId: 'e-1', identityId: 'id-1' });
   });
 });
+
+describe('sendEmail', () => {
+  const OUTGOING = {
+    from: [{ email: 'me@example.com' }],
+    to: [{ email: 'you@example.com' }],
+    subject: 'Hello',
+    textBody: 'Hi',
+  };
+
+  it('reports a failed post-send filing as a warning, not a failed send (B23)', async () => {
+    // Stalwart answers the implicit onSuccessUpdateEmail Email/set with a
+    // method error after the submission was created: the mail went out.
+    mockRequest.mockResolvedValueOnce({
+      methodResponses: [
+        ['Email/set', { created: { draft: { id: 'e-new' } } }, '0'],
+        ['EmailSubmission/set', { created: { 'sub-1': { id: 's-1', sendAt: '2026-09-24T10:00:30Z' } } }, '1'],
+        ['error', { type: 'forbidden', description: 'You do not have access to this mailbox' }, '1'],
+      ],
+    });
+
+    const result = await sendEmail(OUTGOING, 'identity-1', 'sent-mb', 30, { draftsMailboxId: 'drafts-mb' });
+
+    expect(result).toMatchObject({
+      scheduled: true,
+      emailId: 'e-new',
+      emailSubmissionId: 's-1',
+      sendAt: '2026-09-24T10:00:30Z',
+      filingWarning: 'You do not have access to this mailbox',
+    });
+  });
+
+  it('still fails when the submission itself was refused', async () => {
+    mockRequest.mockResolvedValueOnce({
+      methodResponses: [
+        ['Email/set', { created: { draft: { id: 'e-new' } } }, '0'],
+        ['error', { type: 'invalidArguments', description: 'Invalid envelope' }, '1'],
+      ],
+    });
+
+    await expect(sendEmail(OUTGOING, 'identity-1', 'sent-mb', undefined, { draftsMailboxId: 'drafts-mb' }))
+      .rejects.toThrow('Invalid envelope');
+  });
+
+  it('still fails when the message could not be created', async () => {
+    mockRequest.mockResolvedValueOnce({
+      methodResponses: [
+        ['error', { type: 'serverFail', description: 'Disk full' }, '0'],
+        ['EmailSubmission/set', { notCreated: { 'sub-1': { type: 'invalidProperties' } } }, '1'],
+      ],
+    });
+
+    await expect(sendEmail(OUTGOING, 'identity-1', 'sent-mb')).rejects.toThrow('Disk full');
+  });
+});

src/api/email.ts

@@ -1177,6 +1177,15 @@ export async function sendEmail(
   let filingWarning: string | undefined;
   for (const [methodName, result] of res.methodResponses) {
     if (methodName === 'error' || methodName.endsWith('/error')) {
+      // Once the submission exists the message has left (or is held), so a
+      // later error is the implicit `onSuccessUpdateEmail` Email/set that
+      // Stalwart appends failing to file it. Failing the send here made the
+      // user retry and send the message twice.
+      if (emailSubmissionId) {
+        const err = result as { description?: string; type?: string };
+        filingWarning = filingWarning ?? (err.description || err.type || 'post-send filing failed');
+        continue;
+      }
       throw new Error((result as { description?: string }).description ?? 'Send failed');
     }
     if (methodName === 'Email/set') {