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Email clients thread emails by using the message_id metadata. If you want to reply to an email, you should add the In-Reply-To header set to the message_id of the received email. We also recommend setting the subject to start with Re: so that email clients can group the replies together. Here’s an example of replying to an email in a Next.js application:
app/api/events/route.ts
import type { NextRequest } from 'next/server';
import { NextResponse } from 'next/server';
import { Resend } from 'resend';

const resend = new Resend('re_xxxxxxxxx');

export const POST = async (request: NextRequest) => {
  const event = await request.json();

  if (event.type === 'email.received') {
    const { data, error } = await resend.emails.send({
      from: 'Acme <onboarding@resend.dev>',
      to: ['delivered@resend.dev'],
      subject: `Re: ${event.data.subject}`,
      html: '<p>Thanks for your email!</p>',
      headers: {
        'In-Reply-To': event.data.message_id,
      },
      attachments,
    });

    return NextResponse.json(data);
  }

  return NextResponse.json({});
};
If you’re replying multiple times within the same thread, make sure to also append the previous message_ids to the References header, separated by spaces. This helps email clients maintain the correct threading structure. 
const previousReferences = ['<msg_id1@domain.com>', '<msg_id2@domain.com>'];

const { data, error } = await resend.emails.send({
  from: 'Acme <onboarding@resend.dev>',
  to: ['delivered@resend.dev'],
  subject: `Re: ${event.data.subject}`,
  html: '<p>Thanks for your email!</p>',
  headers: {
    'In-Reply-To': event.data.message_id,
    'References': [...previousReferences, event.data.message_id].join(' '),
  },
  attachments,
});