SMTP Test: Send a Test Email and Inspect It

Send a message through the sandbox's SMTP server, then open what arrived: headers, MIME parts, raw source. send.smtp.dev only delivers to accounts in your sandbox, so a test email can't reach a real recipient.

Settings

Create two accounts on the Accounts page: one to send from, one to receive. The examples use sender@test.example.com and qa@test.example.com, and read the sender's password from SMTP_PASSWORD.

From the Command Line

swaks

swaks is the usual SMTP test tool: brew install swaks or apt install swaks. It prints the whole SMTP conversation, so a refused login or recipient shows the server's exact reply.

swaks --server send.smtp.dev --port 587 --tls \
  --auth-user sender@test.example.com --auth-password "$SMTP_PASSWORD" \
  --from sender@test.example.com --to qa@test.example.com \
  --header "Subject: SMTP test" --body "Sent with swaks"

curl

curl speaks SMTP, so there's nothing to install. Write the message to message.txt:

From: sender@test.example.com
To: qa@test.example.com
Subject: SMTP test

Sent with curl.

Then send it:

curl smtp://send.smtp.dev:587 --ssl-reqd \
  --user "sender@test.example.com:$SMTP_PASSWORD" \
  --mail-from sender@test.example.com \
  --mail-rcpt qa@test.example.com \
  --upload-file message.txt

Add -v to see each command and reply.

By Hand with openssl

Typing the SMTP commands yourself shows exactly where a conversation fails. Port 587 starts TLS before login, which telnet can't do, so connect with openssl:

openssl s_client -starttls smtp -connect send.smtp.dev:587 -crlf -quiet

-crlf sends the line endings SMTP expects. -quiet also stops openssl from reading a line that starts with R as a renegotiation command, which would otherwise swallow RCPT TO.

Build the login token in another terminal:

printf '\0%s\0%s' 'sender@test.example.com' "$SMTP_PASSWORD" | base64

Then type one line at a time and wait for each reply:

EHLO localhost
AUTH PLAIN <token>
MAIL FROM:<sender@test.example.com>
RCPT TO:<qa@test.example.com>
DATA
From: sender@test.example.com
To: qa@test.example.com
Subject: SMTP test

Typed by hand.
.
QUIT

The EHLO reply lists what the server supports, including SIZE, its message size limit in bytes. The line with a single . ends the message.

From Your App

Point the app's mailer at the sandbox. Sending from code:

import nodemailer from 'nodemailer'

const transport = nodemailer.createTransport({
  host: 'send.smtp.dev',
  port: 587,
  requireTLS: true,
  auth: {
    user: 'sender@test.example.com',
    pass: process.env.SMTP_PASSWORD,
  },
})

await transport.sendMail({
  from: 'sender@test.example.com',
  to: 'qa@test.example.com',
  subject: 'SMTP test',
  text: 'Sent with Nodemailer',
})

import os
import smtplib
from email.message import EmailMessage

msg = EmailMessage()
msg["From"] = "sender@test.example.com"
msg["To"] = "qa@test.example.com"
msg["Subject"] = "SMTP test"
msg.set_content("Sent with smtplib")

with smtplib.SMTP("send.smtp.dev", 587) as smtp:
    smtp.starttls()
    smtp.login("sender@test.example.com", os.environ["SMTP_PASSWORD"])
    smtp.send_message(msg)

package main

import (
    "net/smtp"
    "os"
)

func main() {
    from := "sender@test.example.com"
    msg := []byte("From: sender@test.example.com\r\n" +
        "To: qa@test.example.com\r\n" +
        "Subject: SMTP test\r\n" +
        "\r\n" +
        "Sent with net/smtp\r\n")

    auth := smtp.PlainAuth("", from, os.Getenv("SMTP_PASSWORD"), "send.smtp.dev")
    err := smtp.SendMail("send.smtp.dev:587", auth, from, []string{"qa@test.example.com"}, msg)
    if err != nil {
        panic(err)
    }
}

Or in your framework's mail settings:

MAIL_MAILER=smtp
MAIL_HOST=send.smtp.dev
MAIL_PORT=587
MAIL_USERNAME=sender@test.example.com
MAIL_PASSWORD=your-account-password
MAIL_FROM_ADDRESS=sender@test.example.com

import os

EMAIL_BACKEND = "django.core.mail.backends.smtp.EmailBackend"
EMAIL_HOST = "send.smtp.dev"
EMAIL_PORT = 587
EMAIL_USE_TLS = True
EMAIL_HOST_USER = "sender@test.example.com"
EMAIL_HOST_PASSWORD = os.environ["SMTP_PASSWORD"]
DEFAULT_FROM_EMAIL = "sender@test.example.com"

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  address: "send.smtp.dev",
  port: 587,
  user_name: "sender@test.example.com",
  password: ENV["SMTP_PASSWORD"],
  authentication: :plain,
  enable_starttls: true
}

spring.mail.host=send.smtp.dev
spring.mail.port=587
spring.mail.username=sender@test.example.com
spring.mail.password=${SMTP_PASSWORD}
spring.mail.properties.mail.smtp.auth=true
spring.mail.properties.mail.smtp.starttls.enable=true
spring.mail.properties.mail.smtp.starttls.required=true

Some frameworks can send a test email for you. Django has python manage.py sendtestemail qa@test.example.com. In Laravel, run php artisan tinker, then Mail::raw('Sent with Laravel', fn ($message) => $message->to('qa@test.example.com')->subject('SMTP test'));.

Read It Back

Open the recipient account in Mail, or log in to webmail.smtp.dev with the account's address and password.

To script it, use the REST API. List the inbox, newest first, then fetch a message by id:

curl "https://api.smtp.dev/accounts/{accountId}/mailboxes/{mailboxId}/messages" \
  -H "X-API-KEY: $SMTP_DEV_API_KEY"

curl "https://api.smtp.dev/accounts/{accountId}/mailboxes/{mailboxId}/messages/{id}/source" \
  -H "X-API-KEY: $SMTP_DEV_API_KEY"

The /source endpoint returns the message exactly as it arrived. The API quickstart covers finding the account and mailbox ids.

curl reads mail over IMAP and POP3 too, with the recipient's own credentials:

# UIDs of the messages in the inbox
curl "imaps://imap.smtp.dev/INBOX" --request "UID SEARCH ALL" \
  --user "qa@test.example.com:$QA_PASSWORD"

# one message, raw
curl "imaps://imap.smtp.dev/INBOX;UID=1" --user "qa@test.example.com:$QA_PASSWORD"

# the first message over POP3
curl "pop3s://pop.smtp.dev/1" --user "qa@test.example.com:$QA_PASSWORD"

What to Check

• Headers. From, Reply-To, Date, and Message-ID are present and say what you intended.
• Both parts. A multipart message has a plain text and an HTML part. Read both. The plain text part is often the one nobody looked at.
• Links. They point at the right environment, with absolute URLs. A localhost link in a staging email is a common bug.
• Attachments. Filename, content type, and size match what the app attached. Size limits covers how encoding grows them.
• Encoding. Non-ASCII names and subjects arrive intact.

When It Fails

Testing Your Own Server

swaks, curl, and openssl work against any SMTP server: change the host, port, and credentials. The sandbox is useful as the far end. Send through your own server or provider to an address on a domain whose MX points at mx.smtp.dev, and the message arrives with SPF, DKIM, and DMARC results next to the raw source.