POST
/
api
/
v2
/
emails
/
{id}
/
reply
curl -X POST https://inbound.new/api/v2/emails/email_abc123/reply \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "support@yourdomain.com",
    "text": "Thank you for your email. We have received your request and will respond shortly."
  }'

{
  "id": "sent_xyz789"
}
This endpoint allows you to reply to emails received through Inbound, maintaining proper email threading with In-Reply-To and References headers.

​
Authentication

This endpoint requires authentication via API key or session token. 
curl -X POST https://inbound.new/api/v2/emails/{id}/reply \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json"

​
Path Parameters

​
id
string
required
The unique identifier of the email you want to reply to. This must be an email that was received through your Inbound account.

​
Request Body

​
from
string
required
The sender email address. Must be in the format "Name <email@domain.com>" or just "email@domain.com". The domain must be verified in your account.
​
to
string | string[]
The recipient email address(es). If not provided, the reply will be sent to the original sender. Can be a single email string or an array of email strings.
​
subject
string
The email subject. If not provided, “Re: ” will be prepended to the original email’s subject.
​
cc
string | string[]
Carbon copy recipient(s). Can be a single email string or an array of email strings.
​
bcc
string | string[]
Blind carbon copy recipient(s). Can be a single email string or an array of email strings.
​
reply_to
string | string[]
Reply-to address(es). Can be a single email string or an array of email strings.
​
text
string
The plain text version of the email body. Either text or html must be provided.
​
html
string
The HTML version of the email body. Either text or html must be provided.
​
headers
object
Custom email headers as key-value pairs. These will be added to the email in addition to the standard headers.
​
attachments
array
Array of file attachments to include with the email.
​
include_original
boolean
default:"true"
Whether to include the original message as a quoted reply. When true, the original message will be appended to your reply with appropriate formatting.

​
Response

​
id
string
required
Unique identifier for the sent reply email

​
Email Threading

This endpoint automatically handles email threading by:
  1. Setting the In-Reply-To header with the Message-ID of the original email
  2. Building the References header to maintain the full conversation thread
  3. Formatting the subject with “Re: ” prefix if not already present
  4. Quoting the original message (when include_original is true) with proper attribution
Email clients like Gmail, Outlook, and Apple Mail will recognize these threading headers and group the emails into a conversation.

​
Examples

​
Basic Reply

Reply to an email with minimal configuration: 
curl -X POST https://inbound.new/api/v2/emails/email_abc123/reply \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "support@yourdomain.com",
    "text": "Thank you for your email. We have received your request and will respond shortly."
  }'

{
  "id": "sent_xyz789"
}

​
Advanced Reply with Custom Recipients

Reply with custom recipients, subject, and HTML content: 
curl -X POST https://inbound.new/api/v2/emails/email_abc123/reply \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "John Doe <john@yourdomain.com>",
    "to": ["customer@example.com"],
    "cc": ["manager@yourdomain.com"],
    "subject": "Re: Support Request #12345",
    "text": "Dear Customer,\n\nWe have resolved your issue...",
    "html": "<p>Dear Customer,</p><p>We have resolved your issue...</p>",
    "headers": {
      "X-Support-Ticket": "12345"
    },
    "include_original": true
  }'

​
Reply Without Quoting Original

Send a reply without including the original message: 
curl -X POST https://inbound.new/api/v2/emails/email_abc123/reply \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "noreply@yourdomain.com",
    "text": "This is an automated response.",
    "include_original": false
  }'

​
Reply with Attachments

Include attachments in your reply: 
const fs = require('fs');

// Read and encode file
const fileContent = fs.readFileSync('invoice.pdf');
const base64Content = fileContent.toString('base64');

const response = await fetch('https://inbound.new/api/v2/emails/email_abc123/reply', {
  method: 'POST',
  headers: {
    'Authorization': 'Bearer YOUR_API_KEY',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    from: 'billing@yourdomain.com',
    text: 'Please find the invoice attached.',
    attachments: [{
      content: base64Content,
      filename: 'invoice.pdf',
      content_type: 'application/pdf'
    }]
  })
});

​
Error Responses

​
error
string
Error message describing what went wrong

​
Common Errors

Status CodeError MessageDescription
400”From address is required”The from field is missing from the request
400”Either html or text content must be provided”No email body content was provided
400”Invalid email format: “One of the email addresses is malformed
401”Unauthorized”Invalid or missing API key
403”You don’t have permission to send from domain: “The sender domain is not verified in your account
404”Email not found”The email ID does not exist or belongs to another user
429”Email sending limit reached”You have exceeded your plan’s email sending limit
500”Internal server error”An unexpected error occurred
{
  "error": "You don't have permission to send from domain: unverified.com"
}

​
Idempotency

This endpoint supports idempotency to prevent duplicate sends. Include an Idempotency-Key header with a unique value: 
curl -X POST https://inbound.new/api/v2/emails/email_abc123/reply \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: unique-key-123" \
  -d '{...}'
If you make multiple requests with the same idempotency key, only the first request will send an email. Subsequent requests will return the same response without sending a duplicate.

​
Best Practices

1

Verify sender domain

Always ensure the from domain is verified in your account before attempting to send replies.
2

Include both text and HTML

For best compatibility, include both text and html versions of your email content.
3

Use idempotency keys

When sending replies programmatically, use idempotency keys to prevent accidental duplicate sends.
4

Handle quoted content carefully

Consider whether including the original message is appropriate for your use case. Automated responses might not need it.
5

Test threading

Test your replies with various email clients to ensure threading works as expected.

​
Rate Limits

Reply sending is subject to the same rate limits as the Send Email endpoint. Your plan determines how many emails you can send per month.
Check your current usage and limits in your account dashboard.