# Reply to Receiving Emails

Email clients thread emails by using the `message_id` metadata.

If you want to reply to an email, add the `In-Reply-To` header set to the `message_id` of the received email. To signal the threading, set the subject to start with `Re:` so that email clients can group the replies together.

## Get the message ID

The `message_id` is included in the [webhook event](/guides/learn-webhooks-emails-received) payload when an email is received:

```json {5} theme={"theme":{"light":"github-light","dark":"vesper"}}
{
  "type": "email.received",
  "data": {
    "email_id": "56761188-7520-42d8-8898-ff6fc54ce618",
    "message_id": "<111-222-333@email.example.com>",
    "subject": "Sending this example",
    "from": "Acme <onboarding@resend.dev>",
    "to": ["delivered@resend.dev"]
  }
}
```

Use the `message_id` value from `event.data.message_id` as the `In-Reply-To` header when sending your reply.

## Send a reply in thread

Here's how you can reply in thread using each SDK:

:::code-group
```ts Node.js {14} theme={"theme":{"light":"github-light","dark":"vesper"}}
import { Resend, type EmailReceivedEvent } from 'resend';

const resend = new Resend('re_xxxxxxxxx');

export async function replyToEmail(event: EmailReceivedEvent) {
  const messageId = event.data.message_id;

  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': messageId,
    },
  });
}
```

```php PHP {13} theme={"theme":{"light":"github-light","dark":"vesper"}}
$resend = Resend::client('re_xxxxxxxxx');

$event = json_decode(file_get_contents('php://input'), true);

$messageId = $event['data']['message_id'];

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

```python Python {14} theme={"theme":{"light":"github-light","dark":"vesper"}}
import resend

resend.api_key = "re_xxxxxxxxx"

def handle_email_received(event: resend.EmailReceivedEvent):
    message_id = event["data"]["message_id"]

    params: resend.Emails.SendParams = {
        "from": "Acme <onboarding@resend.dev>",
        "to": ["delivered@resend.dev"],
        "subject": f"Re: {event['data']['subject']}",
        "html": "<p>Thanks for your email!</p>",
        "headers": {
            "In-Reply-To": message_id,
        },
    }

    email = resend.Emails.send(params)
```

```rb Ruby {13} theme={"theme":{"light":"github-light","dark":"vesper"}}
require "resend"

Resend.api_key = "re_xxxxxxxxx"

message_id = event["data"]["message_id"]

params = {
    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": message_id,
    },
}

sent = Resend::Emails.send(params)
```

```go Go {29} theme={"theme":{"light":"github-light","dark":"vesper"}}
package main

import (
	"fmt"
	"context"

	"github.com/resend/resend-go/v4"
)

func main() {
    ctx := context.TODO()
    client := resend.NewClient("re_xxxxxxxxx")

    var event struct {
        Data struct {
            MessageId string `json:"message_id"`
            Subject   string `json:"subject"`
        } `json:"data"`
    }

    messageId := event.Data.MessageId

    params := &resend.SendEmailRequest{
        From:    "Acme <onboarding@resend.dev>",
        To:      []string{"delivered@resend.dev"},
        Subject: fmt.Sprintf("Re: %s", event.Data.Subject),
        Html:    "<p>Thanks for your email!</p>",
        Headers: map[string]string{
            "In-Reply-To": messageId,
        },
    }

    sent, err := client.Emails.SendWithContext(ctx, params)

    if err != nil {
        panic(err)
    }
    fmt.Println(sent.Id)
}
```

```rust Rust {17} theme={"theme":{"light":"github-light","dark":"vesper"}}
use resend_rs::events::EmailEvent;
use resend_rs::types::CreateEmailBaseOptions;
use resend_rs::{Resend, Result};

async fn reply(event: EmailEvent) -> Result<()> {
    let resend = Resend::new("re_xxxxxxxxx");

    let message_id = &event.data.message_id;
    let subject = format!("Re: {}", event.data.subject);

    let email = CreateEmailBaseOptions::new(
        "Acme <onboarding@resend.dev>",
        ["delivered@resend.dev"],
        &subject,
    )
    .with_html("<p>Thanks for your email!</p>")
    .with_header("In-Reply-To", message_id);

    let _email = resend.emails.send(email).await?;

    Ok(())
}
```

```java Java {20} theme={"theme":{"light":"github-light","dark":"vesper"}}
import com.resend.*;
import com.resend.core.exception.ResendException;
import com.resend.services.emails.model.CreateEmailOptions;
import com.resend.services.emails.model.CreateEmailResponse;
import com.resend.services.receiving.model.ReceivedEmail;
import java.util.Map;

public class Main {
    public static void reply(ReceivedEmail event) throws ResendException {
        Resend resend = new Resend("re_xxxxxxxxx");

        String messageId = event.getMessageId();

        CreateEmailOptions params = CreateEmailOptions.builder()
                .from("Acme <onboarding@resend.dev>")
                .to("delivered@resend.dev")
                .subject("Re: " + event.getSubject())
                .html("<p>Thanks for your email!</p>")
                .headers(Map.of(
                    "In-Reply-To", messageId
                ))
                .build();

        CreateEmailResponse data = resend.emails().send(params);
    }
}
```

```csharp .NET {18} theme={"theme":{"light":"github-light","dark":"vesper"}}
using Resend;
using System.Collections.Generic;

IResend resend = ResendClient.Create( "re_xxxxxxxxx" ); // Or from DI

async Task ReplyAsync(ReceivedEmail eventData)
{
    var messageId = eventData.MessageId;

    var message = new EmailMessage()
    {
        From = "Acme <onboarding@resend.dev>",
        To = "delivered@resend.dev",
        Subject = $"Re: {eventData.Subject}",
        HtmlBody = "<p>Thanks for your email!</p>",
        Headers = new Dictionary<string, string>()
        {
            { "In-Reply-To", messageId },
        },
    };

    var resp = await resend.EmailSendAsync(message);
}
```

```bash cURL {10} theme={"theme":{"light":"github-light","dark":"vesper"}}
curl -X POST 'https://api.resend.com/emails' \
     -H 'Authorization: Bearer re_xxxxxxxxx' \
     -H 'Content-Type: application/json' \
     -d $'{
  "from": "Acme <onboarding@resend.dev>",
  "to": ["delivered@resend.dev"],
  "subject": "Re: Sending this example",
  "html": "<p>Thanks for your email!</p>",
  "headers": {
    "In-Reply-To": "<111-222-333@email.example.com>"
  }
}'
```

```bash CLI {6} theme={"theme":{"light":"github-light","dark":"vesper"}}
resend emails send \
  --from "Acme <onboarding@resend.dev>" \
  --to delivered@resend.dev \
  --subject "Re: Sending this example" \
  --html "<p>Thanks for your email!</p>" \
  --headers "In-Reply-To=<111-222-333@email.example.com>"
```

```rust Rust theme={"theme":{"light":"github-light","dark":"vesper"}}
use axum::{
    extract::State,
    response::{IntoResponse, Json, Response},
};
use resend_rs::{types::CreateEmailBaseOptions, Resend};
use serde::Serialize;
use std::sync::Arc;

struct AppState {
    resend: Resend,
}

#[derive(Serialize)]
struct Empty {}

async fn example(
    State(state): State<Arc<AppState>>,
    Json(event): Json<resend_rs::events::EmailEvent>,
) -> Response {
    if matches!(
        event.r#type,
        resend_rs::events::EmailEventType::EmailReceived
    ) {
        let email = CreateEmailBaseOptions::new(
            "Acme <onboarding@resend.dev>",
            vec!["delivered@resend.dev"],
            format!("Re: {}", event.data.subject),
        )
        .with_html("<p>Thanks for your email!</p>")
        .with_header("In-Reply-To", &event.data.message_id);

        let data = state.resend.emails.send(email).await.unwrap();
        Json(data).into_response()
    } else {
        Json(Empty {}).into_response()
    }
}
```
:::

## Replying multiple times in a thread

If you're replying multiple times within the same thread, make sure to also append
the previous `message_id`s to the `References` header, separated by spaces.
This helps email clients maintain the correct threading structure.

:::code-group
```ts Node.js {14-15} theme={"theme":{"light":"github-light","dark":"vesper"}}
import { Resend, type EmailReceivedEvent } from 'resend';

const resend = new Resend('re_xxxxxxxxx');

export async function replyInThread(event: EmailReceivedEvent) {
  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(' '),
    },
  });
}
```

```php PHP {13-14} theme={"theme":{"light":"github-light","dark":"vesper"}}
$resend = Resend::client('re_xxxxxxxxx');

$event = json_decode(file_get_contents('php://input'), true);

$previousReferences = ['<msg_id1@domain.com>', '<msg_id2@domain.com>'];

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

```python Python {14-15} theme={"theme":{"light":"github-light","dark":"vesper"}}
import resend

resend.api_key = "re_xxxxxxxxx"

def handle_email_received(event: resend.EmailReceivedEvent):
    previous_references = ["<msg_id1@domain.com>", "<msg_id2@domain.com>"]

    params: resend.Emails.SendParams = {
        "from": "Acme <onboarding@resend.dev>",
        "to": ["delivered@resend.dev"],
        "subject": f"Re: {event['data']['subject']}",
        "html": "<p>Thanks for your email!</p>",
        "headers": {
            "In-Reply-To": event["data"]["message_id"],
            "References": " ".join([*previous_references, event["data"]["message_id"]]),
        },
    }

    email = resend.Emails.send(params)
```

```rb Ruby {13-14} theme={"theme":{"light":"github-light","dark":"vesper"}}
require "resend"

Resend.api_key = "re_xxxxxxxxx"

previous_references = ["<msg_id1@domain.com>", "<msg_id2@domain.com>"]

params = {
    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": [*previous_references, event["data"]["message_id"]].join(" "),
    },
}

sent = Resend::Emails.send(params)
```

```go Go {32-33} theme={"theme":{"light":"github-light","dark":"vesper"}}
package main

import (
	"context"
	"fmt"
	"strings"

	"github.com/resend/resend-go/v4"
)

func main() {
    ctx := context.TODO()
    client := resend.NewClient("re_xxxxxxxxx")

    var event struct {
        Data struct {
            MessageId string `json:"message_id"`
            Subject   string `json:"subject"`
        } `json:"data"`
    }

    previousReferences := []string{"<msg_id1@domain.com>", "<msg_id2@domain.com>"}

    allReferences := append(previousReferences, event.Data.MessageId)

    params := &resend.SendEmailRequest{
        From:    "Acme <onboarding@resend.dev>",
        To:      []string{"delivered@resend.dev"},
        Subject: fmt.Sprintf("Re: %s", event.Data.Subject),
        Html:    "<p>Thanks for your email!</p>",
        Headers: map[string]string{
            "In-Reply-To": event.Data.MessageId,
            "References":  strings.Join(allReferences, " "),
        },
    }

    sent, err := client.Emails.SendWithContext(ctx, params)

    if err != nil {
        panic(err)
    }
    fmt.Println(sent.Id)
}
```

```rust Rust {19-20} theme={"theme":{"light":"github-light","dark":"vesper"}}
use resend_rs::events::EmailEvent;
use resend_rs::types::CreateEmailBaseOptions;
use resend_rs::{Resend, Result};

async fn reply(resend: &Resend, event: EmailEvent) -> Result<()> {
    let subject = format!("Re: {}", event.data.subject);
    let previous_references = vec!["<msg_id1@domain.com>", "<msg_id2@domain.com>"];

    let all_references = [previous_references, vec![event.data.message_id.as_str()]]
        .concat()
        .join(" ");

    let email = CreateEmailBaseOptions::new(
        "Acme <onboarding@resend.dev>",
        ["delivered@resend.dev"],
        &subject,
    )
    .with_html("<p>Thanks for your email!</p>")
    .with_header("In-Reply-To", &event.data.message_id)
    .with_header("References", &all_references);

    let _email = resend.emails.send(email).await?;

    Ok(())
}
```

```java Java {25-26} theme={"theme":{"light":"github-light","dark":"vesper"}}
import com.resend.*;
import com.resend.core.exception.ResendException;
import com.resend.services.emails.model.CreateEmailOptions;
import com.resend.services.emails.model.CreateEmailResponse;
import com.resend.services.receiving.model.ReceivedEmail;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class Main {
    public static void reply(ReceivedEmail event) throws ResendException {
        Resend resend = new Resend("re_xxxxxxxxx");

        List<String> previousReferences = List.of("<msg_id1@domain.com>", "<msg_id2@domain.com>");

        List<String> allReferences = new ArrayList<>(previousReferences);
        allReferences.add(event.getMessageId());

        CreateEmailOptions params = CreateEmailOptions.builder()
                .from("Acme <onboarding@resend.dev>")
                .to("delivered@resend.dev")
                .subject("Re: " + event.getSubject())
                .html("<p>Thanks for your email!</p>")
                .headers(Map.of(
                    "In-Reply-To", event.getMessageId(),
                    "References", String.join(" ", allReferences)
                ))
                .build();

        CreateEmailResponse data = resend.emails().send(params);
    }
}
```

```csharp .NET {18-19} theme={"theme":{"light":"github-light","dark":"vesper"}}
using Resend;

IResend resend = ResendClient.Create( "re_xxxxxxxxx" ); // Or from DI

async Task ReplyAsync(ReceivedEmail eventData)
{
    var previousReferences = new List<string> { "<msg_id1@domain.com>", "<msg_id2@domain.com>" };
    previousReferences.Add(eventData.MessageId);

    var message = new EmailMessage()
    {
        From = "Acme <onboarding@resend.dev>",
        To = "delivered@resend.dev",
        Subject = $"Re: {eventData.Subject}",
        HtmlBody = "<p>Thanks for your email!</p>",
        Headers = new Dictionary<string, string>()
        {
            { "In-Reply-To", eventData.MessageId },
            { "References", string.Join(" ", previousReferences) },
        },
    };

    var resp = await resend.EmailSendAsync(message);
}
```

```bash cURL {10-11} theme={"theme":{"light":"github-light","dark":"vesper"}}
curl -X POST 'https://api.resend.com/emails' \
     -H 'Authorization: Bearer re_xxxxxxxxx' \
     -H 'Content-Type: application/json' \
     -d $'{
  "from": "Acme <onboarding@resend.dev>",
  "to": ["delivered@resend.dev"],
  "subject": "Re: Sending this example",
  "html": "<p>Thanks for your email!</p>",
  "headers": {
    "In-Reply-To": "<111-222-333@email.example.com>",
    "References": "<msg_id1@domain.com> <msg_id2@domain.com> <111-222-333@email.example.com>"
  }
}'
```

```bash CLI {6-7} theme={"theme":{"light":"github-light","dark":"vesper"}}
resend emails send \
  --from "Acme <onboarding@resend.dev>" \
  --to delivered@resend.dev \
  --subject "Re: Sending this example" \
  --html "<p>Thanks for your email!</p>" \
  --headers "In-Reply-To=<111-222-333@email.example.com>" \
    "References=<msg_id1@domain.com> <msg_id2@domain.com> <111-222-333@email.example.com>"
```

```rust Rust theme={"theme":{"light":"github-light","dark":"vesper"}}
use resend_rs::{events::EmailEvent, types::CreateEmailBaseOptions, Resend};

async fn reply(resend: &Resend, event: EmailEvent) {
    let previous_references = [
        "<msg_id1@domain.com>",
        "<msg_id2@domain.com>",
        &event.data.message_id,
    ];

    let email = CreateEmailBaseOptions::new(
        "Acme <onboarding@resend.dev>",
        vec!["delivered@resend.dev"],
        format!("Re: {}", event.data.subject),
    )
    .with_html("<p>Thanks for your email!</p>")
    .with_header("In-Reply-To", &event.data.message_id)
    .with_header("References", &previous_references.join(" "));

    let data = resend.emails.send(email).await.unwrap();
}
```
:::

## Related pages

- [Receiving Emails](./learn-dashboard-receiving-introduction.md)
- [Managing Emails](./learn-dashboard-receiving-manage-emails.md)
- [Custom Receiving Domains](./learn-dashboard-receiving-custom-domains.md)
- [Create a receiving Webhook](./learn-dashboard-receiving-create-receiving-webhook.md)
- [Process Receiving Attachments](./learn-dashboard-receiving-attachments.md)
- [Get Email Content](./learn-dashboard-receiving-get-email-content.md)
- [Forward Received Emails](./learn-dashboard-receiving-forward-emails.md)

# Agent Instructions

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