Replying to Emails

Overview

The reply functionality maintains email threading with proper In-Reply-To and References headers, ensuring replies appear in the correct conversation thread.

Basic Reply

Using the SDK Helper

import { Inbound } from 'inboundemail'

const inbound = new Inbound(process.env.INBOUND_API_KEY)

// In your webhook handler
export async function POST(request: NextRequest) {
  const payload = await request.json()
  const { email } = payload
  
  // Simple reply
  await inbound.reply(email, {
    from: '[email protected]',
    text: 'Thanks for your email! We\'ll get back to you soon.'
  })
  
  return NextResponse.json({ success: true })
}

Using the API Directly

// Reply to an email by ID
const reply = await fetch('https://inbound.new/api/e2/emails/email_123/reply', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.INBOUND_API_KEY}`,
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    from: '[email protected]',
    text: 'Thanks for reaching out!'
  })
})

const data = await reply.json()
console.log('Reply sent:', data.id)

Reply with HTML

await inbound.reply(email, {
  from: '[email protected]',
  html: `
    <div style="font-family: Arial, sans-serif;">
      <p>Hi ${email.from?.addresses?.[0]?.name || 'there'},</p>
      <p>Thanks for contacting us! We've received your inquiry about:</p>
      <blockquote style="border-left: 3px solid #ccc; padding-left: 12px; color: #666;">
        ${email.subject}
      </blockquote>
      <p>Our team will review and respond within 24 hours.</p>
      <p>Best regards,<br>Support Team</p>
    </div>
  `,
  text: `
Hi ${email.from?.addresses?.[0]?.name || 'there'},

Thanks for contacting us! We've received your inquiry about: ${email.subject}

Our team will review and respond within 24 hours.

Best regards,
Support Team
  `.trim()
})

How Threading Works

When you reply to an email, Inbound automatically:

Extracts Original Headers

Gets the Message-ID from the original email
Retrieves any existing References headers

Builds Reply Headers

Sets In-Reply-To to the original Message-ID
Appends original Message-ID to References header
Generates new unique Message-ID for the reply

Maintains Subject

Defaults to Re: <original subject>
Or uses your custom subject if provided

This ensures email clients like Gmail, Outlook, and Apple Mail group the messages as a conversation.

Reply Options

Custom Subject

await inbound.reply(email, {
  from: '[email protected]',
  subject: 'Re: Your Support Request #12345',
  text: 'We\'ve created ticket #12345 for your inquiry.'
})

Custom Recipients

By default, replies go to the original sender. Override with:

await inbound.reply(email, {
  from: '[email protected]',
  to: '[email protected]', // Override recipient
  text: 'This reply goes to a different address.'
})

Reply All

Include original CC recipients:

await inbound.reply(email, {
  from: '[email protected]',
  reply_all: true,
  text: 'This reply will include all original recipients.'
})

This automatically adds all original CC recipients to the reply.

With Attachments

await inbound.reply(email, {
  from: '[email protected]',
  html: '<p>Please find the requested document attached.</p>',
  attachments: [
    {
      filename: 'invoice.pdf',
      content: base64Content,
      content_type: 'application/pdf'
    }
  ]
})

With Tags

await inbound.reply(email, {
  from: '[email protected]',
  text: 'Thanks for your message!',
  tags: [
    { name: 'type', value: 'auto-reply' },
    { name: 'original_id', value: email.id }
  ]
})

Real-World Examples

Auto-Reply System

import { Inbound } from 'inboundemail'

const inbound = new Inbound(process.env.INBOUND_API_KEY)

export async function POST(request: NextRequest) {
  const payload = await request.json()
  const { email } = payload
  
  // Check if this is the first email in a thread
  const isNewThread = !email.in_reply_to
  
  if (isNewThread) {
    // Auto-reply to new conversations
    const senderName = email.from?.addresses?.[0]?.name || 'there'
    
    await inbound.reply(email, {
      from: '[email protected]',
      html: `
        <p>Hi ${senderName},</p>
        <p>Thanks for reaching out! We've received your email and will respond within 24 hours.</p>
        <p>In the meantime, you might find these resources helpful:</p>
        <ul>
          <li><a href="https://docs.yourapp.com">Documentation</a></li>
          <li><a href="https://yourapp.com/faq">FAQ</a></li>
          <li><a href="https://status.yourapp.com">Service Status</a></li>
        </ul>
        <p>Best regards,<br>Support Team</p>
      `,
      tags: [{ name: 'type', value: 'auto-reply' }]
    })
  }
  
  return NextResponse.json({ success: true })
}

Smart Reply Based on Content

export async function POST(request: NextRequest) {
  const payload = await request.json()
  const { email } = payload
  
  const subject = email.subject?.toLowerCase() || ''
  const body = email.text_body?.toLowerCase() || ''
  
  // Detect question type
  if (subject.includes('pricing') || body.includes('cost')) {
    await inbound.reply(email, {
      from: '[email protected]',
      html: `
        <p>Thanks for your interest in our pricing!</p>
        <p>You can view our plans here: <a href="https://yourapp.com/pricing">Pricing Page</a></p>
        <p>For custom enterprise quotes, reply to this email or schedule a call.</p>
      `
    })
  } else if (subject.includes('support') || body.includes('help')) {
    await inbound.reply(email, {
      from: '[email protected]',
      html: `
        <p>We've created a support ticket for your inquiry.</p>
        <p>Our team will respond within 24 hours.</p>
      `
    })
  } else {
    await inbound.reply(email, {
      from: '[email protected]',
      html: '<p>Thanks for your email! We\'ll get back to you soon.</p>'
    })
  }
  
  return NextResponse.json({ success: true })
}

Support Ticket Confirmation

async function createTicketAndReply(email: any) {
  // Create ticket in your system
  const ticket = await db.tickets.create({
    subject: email.subject,
    description: email.text_body,
    customer_email: email.from?.addresses?.[0]?.address,
    email_id: email.id
  })
  
  // Send confirmation reply
  await inbound.reply(email, {
    from: '[email protected]',
    html: `
      <div style="font-family: Arial, sans-serif;">
        <h2>Support Ticket Created</h2>
        <p>We've received your inquiry and created ticket <strong>#${ticket.id}</strong>.</p>
        <div style="background: #f5f5f5; padding: 16px; border-radius: 8px; margin: 16px 0;">
          <p style="margin: 0;"><strong>Subject:</strong> ${email.subject}</p>
          <p style="margin: 8px 0 0;"><strong>Status:</strong> Open</p>
        </div>
        <p>Track your ticket: <a href="https://yourapp.com/tickets/${ticket.id}">View Status</a></p>
        <p>Expected response time: Within 24 hours</p>
      </div>
    `,
    tags: [
      { name: 'ticket_id', value: ticket.id },
      { name: 'type', value: 'ticket_confirmation' }
    ]
  })
  
  return ticket
}

Replying to Threads

You can reply to an entire thread (not just a single email) by using the thread ID:

// Reply to a thread (replies to the latest message)
const reply = await fetch('https://inbound.new/api/e2/emails/thread_abc123/reply', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.INBOUND_API_KEY}`,
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    from: '[email protected]',
    text: 'Following up on this conversation...'
  })
})

Learn more about threads in the Email Threads Guide.

Response Format

Successful reply returns:

{
  "id": "sent_xyz789",
  "message_id": "<[email protected]>",
  "aws_message_id": "01020185-1234-5678-9abc-def012345678-000000",
  "replied_to_email_id": "email_abc123",
  "replied_to_thread_id": "thread_def456",
  "is_thread_reply": false
}

Error Handling

try {
  await inbound.reply(email, {
    from: '[email protected]',
    text: 'Thanks for your email!'
  })
} catch (error) {
  if (error.status === 404) {
    console.error('Original email not found')
  } else if (error.status === 403) {
    console.error('Domain not verified or unauthorized')
  } else if (error.status === 400) {
    console.error('Invalid reply data:', error.message)
  } else {
    console.error('Failed to send reply:', error)
  }
}

Important Notes

Domain Verification Required - You can only reply from domains you’ve verified. The special [email protected] address cannot be used for replies.

Blocked Recipients - Replies to addresses that previously bounced will be rejected. The API returns a 400 error listing blocked recipients.

Best Practices

Include both HTML and text for maximum compatibility
Use tags to track reply types and categories
Return 200 from webhooks even if reply fails to prevent retries
Personalize replies using the sender’s name from email.from.addresses[0].name
Check thread status before auto-replying to avoid reply loops
Handle errors gracefully and log failures to your monitoring system

Next Steps

Email Threads

View full conversation threads

Attachments

Include files in your replies

Sending Emails

Learn about sending emails

Webhook Verification

Secure your webhook endpoint

Getting Started

Core Concepts

Guides

SDK

Integrations

Replying to Emails

Overview

Basic Reply

Using the SDK Helper

Using the API Directly

Reply with HTML

How Threading Works

Reply Options

Custom Subject

Custom Recipients

Reply All

With Attachments

With Tags

Real-World Examples

Auto-Reply System

Smart Reply Based on Content

Support Ticket Confirmation

Replying to Threads

Response Format

Error Handling

Important Notes

Best Practices

Next Steps

Email Threads

Attachments

Sending Emails

Webhook Verification

Build docs developers (and LLMs) love

Getting Started

Core Concepts

Guides

SDK

Integrations

​Overview

​Basic Reply

​Using the SDK Helper

​Using the API Directly

​Reply with HTML

​How Threading Works

​Reply Options

​Custom Subject

​Custom Recipients

​Reply All

​With Attachments

​With Tags

​Real-World Examples

​Auto-Reply System

​Smart Reply Based on Content

​Support Ticket Confirmation

​Replying to Threads

​Response Format

​Error Handling

​Important Notes

​Best Practices

​Next Steps

Email Threads

Attachments

Sending Emails

Webhook Verification

Build docs developers (and LLMs) love

Overview

Basic Reply

Using the SDK Helper

Using the API Directly

Reply with HTML

How Threading Works

Reply Options

Custom Subject

Custom Recipients

Reply All

With Attachments

With Tags

Real-World Examples

Auto-Reply System

Smart Reply Based on Content

Support Ticket Confirmation

Replying to Threads

Response Format

Error Handling

Important Notes

Best Practices

Next Steps