Reply to Receiving Emails
Reply to Receiving emails in the same thread.
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
Section titled “Get the message ID”The message_id is included in the webhook event payload when an email is received:
{ "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
Section titled “Send a reply in thread”Here's how you can reply in thread using each SDK:
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: ${event.data.subject}`, html: '<p>Thanks for your email!</p>', headers: { 'In-Reply-To': messageId, }, });}$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, ],]);import resendresend.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)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)package mainimport ( "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)}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(())}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); }}using Resend;using System.Collections.Generic;IResend resend = ResendClient.Create( "re_xxxxxxxxx" ); // Or from DIasync 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);}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>" }}'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>"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
Section titled “Replying multiple times in a thread”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.
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: ${event.data.subject}`, html: '<p>Thanks for your email!</p>', headers: { 'In-Reply-To': event.data.message_id, 'References': [...previousReferences, event.data.message_id].join(' '), }, });}$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']]), ],]);import resendresend.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)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)package mainimport ( "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)}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(())}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); }}using Resend;IResend resend = ResendClient.Create( "re_xxxxxxxxx" ); // Or from DIasync 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);}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>" }}'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>"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();
}