Enhance MailService with provider resilience and delivery status trac…

backend/src/config/env.validation.ts

@@ -48,6 +48,7 @@ export const envValidationSchema = Joi.object({
   MAIL_USER: Joi.string().optional(),
   MAIL_PASS: Joi.string().optional(),
   MAIL_FROM: Joi.string().optional(),
+  MAIL_PROVIDERS: Joi.string().optional(),
 
   // ── Compression (response + request body) ──────────────────────────────────
   COMPRESSION_THRESHOLD: Joi.number().integer().min(0).default(1024).optional(),

backend/src/modules/mail/mail.service.spec.ts

@@ -0,0 +1,62 @@
+import { MailerService } from '@nestjs-modules/mailer';
+import { ConfigService } from '@nestjs/config';
+import { MailService, MailTransport } from './mail.service';
+
+describe('MailService', () => {
+  let mailerService: { sendMail: jest.Mock };
+  let configService: Partial<ConfigService>;
+  let service: MailService;
+
+  beforeEach(() => {
+    mailerService = { sendMail: jest.fn() };
+    configService = {
+      get: jest.fn((key: string, defaultValue?: unknown) => {
+        const values: Record<string, unknown> = {
+          'mail.from': 'noreply@nestera.test',
+        };
+
+        return values[key] ?? defaultValue;
+      }),
+    };
+
+    service = new MailService(
+      mailerService as unknown as MailerService,
+      configService as ConfigService,
+      () => [],
+    );
+  });
+
+  it('retries the primary provider and falls back to the next provider when it is unavailable', async () => {
+    const primaryProvider: MailTransport = {
+      name: 'primary',
+      maxRetries: 1,
+      retryDelayMs: 0,
+      sendMail: jest.fn().mockRejectedValue(new Error('primary outage')),
+    };
+
+    const secondaryProvider: MailTransport = {
+      name: 'secondary',
+      maxRetries: 0,
+      retryDelayMs: 0,
+      sendMail: jest.fn().mockResolvedValue({ messageId: 'secondary-id' }),
+    };
+
+    service = new MailService(
+      mailerService as unknown as MailerService,
+      configService as ConfigService,
+      () => [primaryProvider, secondaryProvider],
+    );
+
+    await expect(
+      service.sendRawMail('user@example.com', 'Test subject', 'hello world'),
+    ).resolves.toBeUndefined();
+
+    expect(primaryProvider.sendMail).toHaveBeenCalledTimes(2);
+    expect(secondaryProvider.sendMail).toHaveBeenCalledTimes(1);
+
+    const deliveryStatuses = service.getDeliveryStatusHistory();
+    expect(deliveryStatuses).toHaveLength(1);
+    expect(deliveryStatuses[0].status).toBe('sent');
+    expect(deliveryStatuses[0].provider).toBe('secondary');
+  });
+});