fix: send Buffer for Outlook sendMail base64 payload to avoid JSON quoting

String payloads passed to OutlookOauth.request() go through
JSON.stringify(), wrapping the base64 body in quotes. Microsoft Graph
expects raw base64 with Content-Type: text/plain. Wrapping the base64
string in Buffer.from() routes it through the Buffer code path which
sends the body as-is.

Author: andris9

lib/email-client/outlook-client.js

@@ -2287,7 +2287,7 @@ class OutlookClient extends BaseClient {
             // Use raw MIME format (default)
             // Preserves calendar invites and special MIME types, but ignores from field
             try {
-                await this.request(`/${this.oauth2UserPath}/sendMail`, 'post', raw.toString('base64'), {
+                await this.request(`/${this.oauth2UserPath}/sendMail`, 'post', Buffer.from(raw.toString('base64')), {
                     contentType: 'text/plain',
                     returnText: true
                 });

test/oauth-request-dispatch-test.js

@@ -52,6 +52,34 @@ async function stopServer(server) {
     await new Promise(resolve => server.close(resolve));
 }
 
+/**
+ * Creates and starts a test HTTP server that captures the raw request body.
+ *
+ * @returns {Promise<{server: http.Server, baseUrl: string, getBody: () => Buffer}>}
+ */
+async function startBodyCapturingServer() {
+    let capturedBody = null;
+
+    const server = http.createServer((req, res) => {
+        const chunks = [];
+        req.on('data', chunk => chunks.push(chunk));
+        req.on('end', () => {
+            capturedBody = Buffer.concat(chunks);
+            res.writeHead(200, { 'Content-Type': 'application/json' });
+            res.end(JSON.stringify({ ok: true }));
+        });
+    });
+
+    await new Promise(resolve => server.listen(0, '127.0.0.1', resolve));
+    const { port } = server.address();
+
+    return {
+        server,
+        baseUrl: `http://127.0.0.1:${port}`,
+        getBody: () => capturedBody
+    };
+}
+
 test('Buffer payload dispatcher and Gmail endpoint selection', async t => {
     t.after(() => {
         setTimeout(() => process.exit(), 1000).unref();
@@ -375,4 +403,46 @@ test('Buffer payload dispatcher and Gmail endpoint selection', async t => {
 
         assert.ok(totalJsonBody < GMAIL_JSON_LIMIT, `Max JSON body (${totalJsonBody} bytes) should be under Gmail 5MB limit (${GMAIL_JSON_LIMIT} bytes)`);
     });
+
+    // ---------------------------------------------------------------
+    // Fix 3: Outlook sendMail base64 body must not be JSON-quoted.
+    //
+    // When outlook-client.js sends a base64-encoded MIME message via
+    // sendMail, the payload must arrive as a raw base64 string with
+    // Content-Type: text/plain. If the payload is passed as a JS string
+    // instead of a Buffer, the non-Buffer branch in outlook.js wraps it
+    // with JSON.stringify(), adding quotes around the base64 content.
+    // The fix wraps the base64 string in a Buffer so it takes the
+    // Buffer code path, which sends the body as-is.
+    // ---------------------------------------------------------------
+
+    await t.test('Outlook: Buffer payload with text/plain contentType is sent without JSON quoting', async () => {
+        const { server, baseUrl, getBody } = await startBodyCapturingServer();
+        try {
+            const outlook = new OutlookOauth({
+                clientId: 'test-id',
+                clientSecret: 'test-secret',
+                authority: 'common',
+                redirectUrl: 'http://localhost/callback',
+                setFlag: async () => {}
+            });
+
+            const base64Content = Buffer.from('From: a@b.com\r\nTo: c@d.com\r\nSubject: Test\r\n\r\nHello').toString('base64');
+            const payload = Buffer.from(base64Content);
+
+            await outlook.request('fake-token', `${baseUrl}/me/sendMail`, 'post', payload, {
+                contentType: 'text/plain',
+                returnText: true
+            });
+
+            const receivedBody = getBody().toString();
+
+            // The body must be the raw base64 string, not wrapped in JSON quotes
+            assert.strictEqual(receivedBody, base64Content, 'Body should be raw base64, not JSON-stringified');
+            assert.ok(!receivedBody.startsWith('"'), 'Body must not start with a JSON quote');
+            assert.ok(!receivedBody.endsWith('"'), 'Body must not end with a JSON quote');
+        } finally {
+            await stopServer(server);
+        }
+    });
 });