feat(providers): forward email headers across all providers for reply threading fixes NV-7402

packages/providers/src/lib/email/braze/braze.provider.ts

@@ -69,7 +69,7 @@ export class BrazeEmailProvider extends BaseProvider implements IEmailProvider {
           bcc: options.bcc?.join(','),
           plaintext_body: options.text || null,
           extras: options.payloadDetails || {},
-          headers: {},
+          headers: options.headers && Object.keys(options.headers).length > 0 ? options.headers : {},
           should_inline_css: true,
           attachments: [],
         },

packages/providers/src/lib/email/emailjs/emailjs.provider.ts

@@ -29,6 +29,7 @@ export class EmailJsProvider extends BaseProvider implements IEmailProvider {
     await this.ensureClientInitialized();
 
     const headers: Message['header'] = {
+      ...emailOptions.headers,
       from: emailOptions.from || this.config.from,
       to: emailOptions.to,
       subject: emailOptions.subject,

packages/providers/src/lib/email/mailersend/mailersend.provider.ts

@@ -76,6 +76,14 @@ export class MailersendEmailProvider extends BaseProvider implements IEmailProvi
       emailParams.setReplyTo(replyTo);
     }
 
+    const inReplyTo = Object.entries(options.headers ?? {}).find(
+      ([headerName]) => headerName.toLowerCase() === 'in-reply-to'
+    )?.[1];
+
+    if (inReplyTo) {
+      emailParams.setInReplyTo(inReplyTo);
+    }
+
     return emailParams;
   }
 

packages/providers/src/lib/email/mailgun/mailgun.provider.spec.ts

@@ -33,6 +33,35 @@ test('should trigger mailgun correctly', async () => {
   api.done();
 });
 
+test('should forward custom headers as h: prefixed fields', async () => {
+  const provider = new MailgunEmailProvider(mockConfig);
+
+  const api = nock('https://api.mailgun.net');
+
+  api
+    .post('/v3/test.com/messages', (body) => {
+      expect(body.includes('name="h:In-Reply-To"')).toBeTruthy();
+      expect(body.includes('name="h:References"')).toBeTruthy();
+
+      return true;
+    })
+    .reply(200, {
+      message: 'Queued. Thank you.',
+      id: '<20111114174239.25659.5817@samples.mailgun.org>',
+    });
+
+  await provider.sendMessage({
+    ...mockNovuMessage,
+    headers: {
+      'In-Reply-To': '<original-message-id@example.com>',
+      References: '<original-message-id@example.com>',
+    },
+  });
+
+  expect(api.isDone()).toBeTruthy();
+  api.done();
+});
+
 test('should trigger mailgun correctly with custom baseUrl', async () => {
   const provider = new MailgunEmailProvider({
     ...mockConfig,

packages/providers/src/lib/email/mailgun/mailgun.provider.ts

@@ -106,6 +106,16 @@ export class MailgunEmailProvider extends BaseProvider implements IEmailProvider
         }),
     };
 
+    if (emailOptions.headers) {
+      for (const [key, value] of Object.entries(emailOptions.headers)) {
+        if (emailOptions.replyTo && key.toLowerCase() === 'reply-to') {
+          continue;
+        }
+
+        data[`h:${key}`] = value;
+      }
+    }
+
     if (emailOptions.replyTo) {
       data['h:Reply-To'] = emailOptions.replyTo;
     }

packages/providers/src/lib/email/mailjet/mailjet.provider.ts

@@ -118,6 +118,7 @@ export class MailjetEmailProvider extends BaseProvider implements IEmailProvider
           Base64Content: attachment.file.toString('base64'),
           ContentID: attachment.cid,
         })),
+      ...(options.headers && Object.keys(options.headers).length > 0 && { Headers: options.headers }),
     }).body;
 
     if (options.replyTo) {

packages/providers/src/lib/email/mandrill/mandril.interface.ts

@@ -15,6 +15,7 @@ interface IMandrillSendOptionsMessage {
   html: string;
   to: { email: string; type: 'to' | string }[];
   attachments: IMandrillAttachment[];
+  headers?: Record<string, string>;
 }
 interface IMandrillTemplateSendOptionsMessage extends IMandrillSendOptionsMessage {
   global_merge_vars?: { name: string; content: string }[];

packages/providers/src/lib/email/mandrill/mandrill.provider.spec.ts

@@ -47,6 +47,53 @@ test('should send a standard email through Mandrill', async () => {
   });
 });
 
+test('should forward custom headers in message.headers', async () => {
+  const provider = new MandrillProvider(mockConfig);
+  const spy = vi.spyOn(provider['transporter'].messages, 'send').mockImplementation(async () => {
+
+    return [{}] as any;
+  });
+
+  await provider.sendMessage({
+    to: ['test2@test.com'],
+    subject: 'test subject',
+    html: '<div> Mail Content </div>',
+    headers: {
+      'In-Reply-To': '<original-message-id@example.com>',
+      References: '<original-message-id@example.com>',
+    },
+  });
+
+  expect(spy).toHaveBeenCalledWith(
+    expect.objectContaining({
+      message: expect.objectContaining({
+        headers: {
+          'In-Reply-To': '<original-message-id@example.com>',
+          References: '<original-message-id@example.com>',
+        },
+      }),
+    })
+  );
+});
+
+test('should not add headers to message when no custom headers provided', async () => {
+  const provider = new MandrillProvider(mockConfig);
+  const spy = vi.spyOn(provider['transporter'].messages, 'send').mockImplementation(async () => {
+
+    return [{}] as any;
+  });
+
+  await provider.sendMessage({
+    to: ['test2@test.com'],
+    subject: 'test subject',
+    html: '<div> Mail Content </div>',
+  });
+
+  const payload = spy.mock.calls[0][0];
+
+  expect(payload.message).not.toHaveProperty('headers');
+});
+
 test('should send an email using a Mandrill template', async () => {
   const provider = new MandrillProvider(mockConfig);
   const spy = vi.spyOn(provider['transporter'].messages, 'sendTemplate').mockImplementation(async () => {

packages/providers/src/lib/email/mandrill/mandrill.provider.ts

@@ -86,6 +86,8 @@ export class MandrillProvider extends BaseProvider implements IEmailProvider {
         type: attachment.mime,
         name: attachment?.name,
       })),
+      ...(emailOptions.headers &&
+        Object.keys(emailOptions.headers).length > 0 && { headers: emailOptions.headers }),
     };
 
     const { customData } = emailOptions;

packages/providers/src/lib/email/netcore/netcore.provider.ts

@@ -93,6 +93,10 @@ export class NetCoreProvider extends BaseProvider implements IEmailProvider {
       });
     }
 
+    if (options.headers && Object.keys(options.headers).length > 0) {
+      data.personalizations[0].headers = options.headers;
+    }
+
     const emailOptions = {
       method: 'POST',
       url: '/mail/send',

packages/providers/src/lib/email/nodemailer/nodemailer.provider.spec.ts

@@ -204,3 +204,48 @@ describe.skip('NodemailerProvider', () => {
     });
   });
 });
+
+describe('NodemailerProvider header forwarding', () => {
+  const mockConfig = {
+    host: 'test.test.email',
+    port: 587,
+    secure: false,
+    from: 'test@test.com',
+    senderName: 'John Doe',
+    user: 'test@test.com',
+    password: 'test123',
+  };
+
+  test('should forward custom headers to sendMail', async () => {
+    const provider = new NodemailerProvider(mockConfig);
+    const spy = vi.spyOn(provider['transports'], 'sendMail').mockResolvedValue({ messageId: 'test-id' } as any);
+
+    await provider.sendMessage({
+      ...mockNovuMessage,
+      headers: {
+        'In-Reply-To': '<original-message-id@example.com>',
+        References: '<original-message-id@example.com>',
+      },
+    });
+
+    expect(spy).toHaveBeenCalledWith(
+      expect.objectContaining({
+        headers: {
+          'In-Reply-To': '<original-message-id@example.com>',
+          References: '<original-message-id@example.com>',
+        },
+      })
+    );
+  });
+
+  test('should not include headers field when no custom headers provided', async () => {
+    const provider = new NodemailerProvider(mockConfig);
+    const spy = vi.spyOn(provider['transports'], 'sendMail').mockResolvedValue({ messageId: 'test-id' } as any);
+
+    await provider.sendMessage(mockNovuMessage);
+
+    const payload = spy.mock.calls[0][0] as Record<string, unknown>;
+
+    expect(payload).not.toHaveProperty('headers');
+  });
+});

packages/providers/src/lib/email/nodemailer/nodemailer.provider.ts

@@ -153,6 +153,10 @@ export class NodemailerProvider extends BaseProvider implements IEmailProvider {
       sendMailOptions.replyTo = options.replyTo;
     }
 
+    if (options.headers && Object.keys(options.headers).length > 0) {
+      sendMailOptions.headers = options.headers;
+    }
+
     return sendMailOptions;
   }
 }

packages/providers/src/lib/email/outlook365/outlook365.provider.ts

@@ -97,6 +97,10 @@ export class Outlook365Provider extends BaseProvider implements IEmailProvider {
       sendMailOptions.replyTo = options.replyTo;
     }
 
+    if (options.headers && Object.keys(options.headers).length > 0) {
+      sendMailOptions.headers = options.headers;
+    }
+
     return sendMailOptions;
   }
 }

packages/providers/src/lib/email/postmark/postmark.provider.spec.ts

@@ -62,6 +62,45 @@ test('should trigger postmark correctly', async () => {
   });
 });
 
+test('should forward custom headers in Postmark Headers format', async () => {
+  const provider = new PostmarkEmailProvider(mockConfig);
+  const spy = vi.spyOn((provider as any).client, 'sendEmail').mockImplementation(async () => {
+    return {};
+  });
+
+  await provider.sendMessage({
+    ...mockNovuMessage,
+    headers: {
+      'In-Reply-To': '<original-message-id@example.com>',
+      References: '<original-message-id@example.com>',
+    },
+  });
+
+  expect(spy).toHaveBeenCalledWith(
+    expect.objectContaining({
+      Headers: [
+        { Name: 'In-Reply-To', Value: '<original-message-id@example.com>' },
+        { Name: 'References', Value: '<original-message-id@example.com>' },
+      ],
+    })
+  );
+});
+
+test('should not add Headers field when no custom headers provided', async () => {
+  const provider = new PostmarkEmailProvider(mockConfig);
+  const spy = vi.spyOn((provider as any).client, 'sendEmail').mockImplementation(async () => {
+    return {};
+  });
+
+  await provider.sendMessage(mockNovuMessage);
+
+  expect(spy).toHaveBeenCalledWith(
+    expect.not.objectContaining({
+      Headers: expect.anything(),
+    })
+  );
+});
+
 test('should trigger postmark correctly with _passthrough', async () => {
   const provider = new PostmarkEmailProvider(mockConfig);
   const spy = vi.spyOn((provider as any).client, 'sendEmail').mockImplementation(async () => {

packages/providers/src/lib/email/postmark/postmark.provider.ts

@@ -90,6 +90,10 @@ export class PostmarkEmailProvider extends BaseProvider implements IEmailProvide
       mailData.ReplyTo = options.replyTo;
     }
 
+    if (options.headers && Object.keys(options.headers).length > 0) {
+      mailData.Headers = Object.entries(options.headers).map(([Name, Value]) => ({ Name, Value }));
+    }
+
     return mailData;
   }
 

packages/providers/src/lib/email/ses/ses.provider.spec.ts

@@ -107,6 +107,30 @@ test('should trigger ses library correctly', async () => {
   expect(response.id).toEqual('<mock-message-id@email.amazonses.com>');
 });
 
+test('should forward custom headers in raw email content', async () => {
+  const mockResponse = { MessageId: 'mock-message-id' };
+  const spy = vi.spyOn(SESv2Client.prototype, 'send').mockImplementation(async () => {
+
+    return mockResponse as any;
+  });
+
+  const provider = new SESEmailProvider(mockConfig);
+  await provider.sendMessage({
+    ...mockNovuMessage,
+    headers: {
+      'In-Reply-To': '<original-message-id@example.com>',
+      References: '<original-message-id@example.com>',
+    },
+  });
+
+  const bufferArray = spy.mock.calls[0][0].input['Content']['Raw']['Data'];
+  const emailContent = Buffer.from(bufferArray).toString();
+
+  expect(spy).toHaveBeenCalled();
+  expect(emailContent.includes('In-Reply-To: <original-message-id@example.com>')).toBe(true);
+  expect(emailContent.includes('References: <original-message-id@example.com>')).toBe(true);
+});
+
 test('should trigger ses library correctly with _passthrough', async () => {
   const mockResponse = { MessageId: 'mock-message-id' };
   const spy = vi.spyOn(SESv2Client.prototype, 'send').mockImplementation(async () => {

packages/providers/src/lib/email/ses/ses.provider.ts

@@ -34,7 +34,7 @@ export class SESEmailProvider extends BaseProvider implements IEmailProvider {
   }
 
   private async sendMail(
-    { html, text, to, from, senderName, subject, attachments, cc, bcc, replyTo },
+    { html, text, to, from, senderName, subject, attachments, cc, bcc, replyTo, headers = {} },
     bridgeProviderData: WithPassthrough<Record<string, unknown>> = {}
   ) {
     const transporter = nodemailer.createTransport({
@@ -54,6 +54,7 @@ export class SESEmailProvider extends BaseProvider implements IEmailProvider {
       cc,
       bcc,
       replyTo,
+      ...(headers && Object.keys(headers).length > 0 && { headers }),
       ...(this.config.configurationSetName && {
         ses: { ConfigurationSetName: this.config.configurationSetName },
       }),
@@ -63,7 +64,7 @@ export class SESEmailProvider extends BaseProvider implements IEmailProvider {
   }
 
   async sendMessage(
-    { html, text, to, from, subject, attachments, cc, bcc, replyTo, senderName }: IEmailOptions,
+    { html, text, to, from, subject, attachments, cc, bcc, replyTo, senderName, headers }: IEmailOptions,
     bridgeProviderData: WithPassthrough<Record<string, unknown>> = {}
   ): Promise<ISendMessageSuccessResponse> {
     const info = await this.sendMail(
@@ -84,6 +85,7 @@ export class SESEmailProvider extends BaseProvider implements IEmailProvider {
         cc,
         bcc,
         replyTo,
+        headers,
       },
       bridgeProviderData
     );