At Valyent, we are building open-source software for developers.
As part of this mission, we're currently developing Ferdinand, our email sending service for developers (currently in alpha).
Email infrastructure relies on several key protocols, with the most important being:
In today’s article, we're going to focus on building our own outbound SMTP server, mirroring the approach we've taken with Ferdinand. By doing so, we'll gain a deep understanding of the most crucial component in email sending infrastructure.
"What I cannot create, I do not understand."
— Richard Feynman
By building an outbound SMTP server from scratch, you can gain a level of insight into email delivery that most developers never achieve.
To proceed, we are going to use the Go programming language, along with the awesome mail libraries from Simon Ser. We'll demystify the process, show you how to send emails to other servers, and even explain key concepts like SPF, DKIM, and DMARC allowing for deliverability.
By the end, you'll have at least have a deeper understanding of email infrastructure, despite not having a production-ready SMTP server.
Before we dive into the code, let's review what SMTP is and how it works. SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending email across the Internet. It's a relatively simple, text-based protocol that operates on a client-server model.
The SMTP protocol makes use of commands. Each command in SMTP serves a specific purpose in the email transmission process. They allow servers to introduce themselves, specify senders and recipients, transfer the actual email content, and manage the overall communication session. Think of these commands as a structured conversation between two email servers, where each command represents a specific statement or question in that conversation.
When you build an SMTP server, you're essentially creating a program that can speak this language fluently, interpreting incoming commands and responding appropriately, as well as issuing the right commands when sending emails.
Let's explore the most important SMTP commands to see how this conversation unfolds:
DATA From: john@example.com To: jane@example.com Subject: Hello This is the body of the email. .
A typical SMTP conversation might look like this:
C: EHLO client.example.com S: 250-smtp.example.com Hello client.example.com S: 250-SIZE 14680064 S: 250-AUTH LOGIN PLAIN S: 250 HELP C: MAIL FROM:<sender@example.com> S: 250 OK C: RCPT TO:<recipient@example.com> S: 250 OK C: DATA S: 354 Start mail input; end with <CRLF>.<CRLF> C: From: sender@example.com C: To: recipient@example.com C: Subject: Test Email C: C: This is a test email. C: . S: 250 OK: queued as 12345 C: QUIT S: 221 Bye
Authentication is a crucial aspect of SMTP, especially for outbound email servers. It helps prevent unauthorized use of the server and reduces spam. There are several authentication methods used in SMTP:
Here's an example of how PLAIN authentication looks in an SMTP conversation:
C: EHLO example.com S: 250-STARTTLS S: 250 AUTH PLAIN LOGIN C: AUTH PLAIN AGVtYWlsQGV4YW1wbGUuY29tAHBhc3N3b3Jk S: 235 2.7.0 Authentication successful
In this example, AGVtYWlsQGV4YW1wbGUuY29tAHBhc3N3b3Jk is the base64-encoded version of \0email@example.com\0password.
When implementing authentication in your SMTP server, you'll need to:
Now, let's move on to implementing these concepts in our Go SMTP server.
Imagine sending a letter through the postal service without a return address or an official stamp. It might reach its destination, but there's a good chance it'll end up in the "suspicious mail" pile. In the digital world of email, we face a similar challenge.
How do we ensure our emails aren't just sent, but actually delivered and trusted?
Enter the holy trinity of email authentication: DKIM, SPF, and DMARC.
DKIM (DomainKeys Identified Mail) is like a wax seal on a medieval letter. It proves the email hasn't been tampered with during transit.
How it works:
Think of it as your email's passport, stamped and verified at each checkpoint.
Example DKIM DNS Record:
<selector>._domainkey.<domain>.<tld>. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC3QEKyU1fSma0axspqYK5iAj+54lsAg4qRRCnpKK68hawSd8zpsDz77ntGCR0X2mHVvkHbX6dX...oIDAQAB"
Here, 'selector' is a unique identifier for this DKIM key, and the long string is your public key.
SPF (Sender Policy Framework) is like the bouncer at an exclusive club. It specifies which email servers are allowed to send emails on behalf of your domain.
How it works:
It's like saying, "If the email didn't come from one of these guys, it's not with us!"
Example SPF DNS Record:
<domain>.<tld>. IN TXT "v=spf1 ip4:192.0.2.0/24 include:_spf.google.com ~all"
This record says:
DMARC (Domain-based Message Authentication, Reporting & Conformance) is the wise judge that decides what happens to emails that fail DKIM or SPF checks.
How it works:
Think of DMARC as your email bouncer's rulebook and incident report.
Example DMARC DNS Record:
_dmarc.<domain>.<tld>. IN TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc-reports@<domain>.<tld>"
This record says:
Together, DKIM, SPF, and DMARC form a powerful shield against email spoofing and phishing. They tell receiving servers, "This email is really from us, sent by someone we trust, and here's what to do if something seems fishy."
Implementing this trinity not only improves your email deliverability but also protects your domain's reputation. It's like having a state-of-the-art security system for your email infrastructure.
As we build our SMTP server, keeping these authentication methods in mind will be crucial for ensuring our emails don't just get sent, but actually reach their destination and are trusted when they arrive. Remember, when implementing these records on a production domain, start with permissive policies and gradually tighten them as you confirm everything is working correctly.
First, let's create a new directory for our project and initialize a Go module:
mkdir go-smtp-server cd go-smtp-server go mod init github.com/yourusername/go-smtp-server
We'll need a few dependencies for our SMTP server. Run the following commands:
go get github.com/emersion/go-smtp go get github.com/emersion/go-sasl go get github.com/emersion/go-msgauth
package main
import (
"log"
"time"
"io"
"github.com/emersion/go-smtp"
)
func main() {
s := smtp.NewServer(&Backend{})
s.Addr = ":2525"
s.Domain = "localhost"
s.WriteTimeout = 10 * time.Second
s.ReadTimeout = 10 * time.Second
s.MaxMessageBytes = 1024 * 1024
s.MaxRecipients = 50
s.AllowInsecureAuth = true
log.Println("Starting server at", s.Addr)
if err := s.ListenAndServe(); err != nil {
log.Fatal(err)
}
}
// Backend implements SMTP server methods.
type Backend struct{}
func (bkd *Backend) NewSession(_ *smtp.Conn) (smtp.Session, error) {
return &Session{}, nil
}
// A Session is returned after EHLO.
type Session struct{}
// We'll implement the Session methods next
This creates an SMTP server, listening on the 2525 port, a convenient choice for development purposes, since this port doesn’t require administrative privileges, unlike the standard ports 25 (standard SMTP), 465 (TLS), 587 (STARTTLS).
The EHLO/HELO command is handled automatically by the go-smtp library. We don't need to implement it ourselves.
Add this method to the Session struct:
func (s *Session) Mail(from string, opts *smtp.MailOptions) error {
fmt.Println("Mail from:", from) s.From = from
return nil
}
This method is called when the server receives a MAIL FROM command. It logs the sender's address and stores it in the session.
Add this method to the Session struct:
func (s *Session) Rcpt(to string) error {
fmt.Println("Rcpt to:", to)
s.To = append(s.To, to)
return nil
}
This method is called for each RCPT TO command. It logs the recipient's address and adds it to the list of recipients for this session.
Add this method to the Session struct:
import (
"fmt"
"io"
)
func (s *Session) Data(r io.Reader) error {
if b, err := io.ReadAll(r); err != nil {
return err
} else {
fmt.Println("Received message:", string(b))
// Here you would typically process the email
return nil
}
}
This method is called when the server receives the DATA command. It reads the entire email message and logs it. In a real server, you would process the email here.
Add this method to the Session struct:
func (s *Session) AuthPlain(username, password string) error {
if username != "testuser" || password != "testpass" {
return fmt.Errorf("Invalid username or password")
}
return nil
}
This implements a basic authentication mechanism. Note that this is for demonstration purposes only and should not be used in production.
Add this method to the Session struct:
func (s *Session) Reset() {
s.From = "" s.To = []string{}
}
This method is called when the server receives a RSET command. It resets the session state.
Add this method to the Session struct:
func (s *Session) Logout() error {
return nil
}
This method is called when the server receives a QUIT command. In this simple implementation, we don't need to do anything special.
Once we've received and processed an email, the next step is to send it to its destination. This involves two key steps: finding the recipient's mail server using MX (Mail Exchanger) records, and attempting to send the email using standard SMTP ports.
First, let's add a function to look up MX records:
import "net"
func lookupMX(domain string) ([]*net.MX, error) {
mxRecords, err := net.LookupMX(domain)
if err != nil {
return nil, fmt.Errorf("Error looking up MX records: %v", err)
}
return mxRecords, nil
}
Next, let's create a function that attempts to send an email using different ports:
import (
"crypto/tls"
"net/smtp"
"strings"
)
func sendMail(from string, to string, data []byte) error {
domain := strings.Split(to, "@")[1]
mxRecords, err := lookupMX(domain)
if err != nil {
return err
}
for _, mx := range mxRecords {
host := mx.Host
for _, port := range []int{25, 587, 465} {
address := fmt.Sprintf("%s:%d", host, port)
var c *smtp.Client
var err error
switch port {
case 465:
// SMTPS
tlsConfig := &tls.Config{ServerName: host}
conn, err := tls.Dial("tcp", address, tlsConfig)
if err != nil {
continue
}
c, err = smtp.NewClient(conn, host)
case 25, 587:
// SMTP or SMTP with STARTTLS
c, err = smtp.Dial(address)
if err != nil {
continue
}
if port == 587 {
if err = c.StartTLS(&tls.Config{ServerName: host}); err != nil {
c.Close()
continue
}
}
}
if err != nil {
continue
}
// SMTP conversation
if err = c.Mail(from); err != nil {
c.Close()
continue
}
if err = c.Rcpt(to); err != nil {
c.Close()
continue
}
w, err := c.Data()
if err != nil {
c.Close()
continue
}
if _, err := w.Write(data); err != nil {
c.Close()
continue
}
err = w.Close()
if err != nil {
c.Close()
continue
}
c.Quit()
return nil
}
}
return fmt.Errorf("Failed to send email to %s", to)
}
This function does the following:
Now, let's modify our Data method in the Session struct to use this new sendMail function:
func (s *Session) Data(r io.Reader) error {
if data, err := io.ReadAll(r); err != nil {
return err
} else {
fmt.Println("Received message:", string(data))
for _, recipient := range s.To {
if err := sendMail(s.From, recipient, data); err != nil {
fmt.Printf("Failed to send email to %s: %v", recipient, err)
} else {
fmt.Printf("Email sent successfully to %s", recipient)
}
}
return nil
}
}
This implementation will attempt to send the received email to each recipient using the appropriate mail server and port.
Now, let's add DKIM signing to our email sending process. First, we need to import the necessary packages and set up our DKIM options:
import (
// ... other imports ...
"crypto/rsa"
"crypto/x509"
"encoding/pem"
"github.com/emersion/go-msgauth/dkim"
)
// Load your DKIM private key
var dkimPrivateKey *rsa.PrivateKey
func init() {
// Load your DKIM private key from a file
privateKeyPEM, err := ioutil.ReadFile("path/to/your/private_key.pem")
if err != nil {
log.Fatalf("Failed to read private key: %v", err)
}
block, _ := pem.Decode(privateKeyPEM)
if block == nil {
log.Fatalf("Failed to parse PEM block containing the private key")
}
privateKey, err := x509.ParsePKCS1PrivateKey(block.Bytes)
if err != nil {
log.Fatalf("Failed to parse private key: %v", err)
}
dkimPrivateKey = privateKey
}
// DKIM options
var dkimOptions = &dkim.SignOptions{
Domain: "example.com",
Selector: "default",
Signer: dkimPrivateKey,
}
Next, let's modify our sendMail function to include DKIM signing:
func sendMail(from string, to string, data []byte) error {
// ... [previous MX lookup code] ...
for _, mx := range mxRecords {
host := mx.Host
for _, port := range []int{25, 587, 465} {
// ... [previous connection code] ...
// DKIM sign the message
var b bytes.Buffer
if err := dkim.Sign(&b, bytes.NewReader(data), dkimOptions); err != nil {
return fmt.Errorf("Failed to sign email with DKIM: %v", err)
}
signedData := b.Bytes()
// SMTP conversation
if err = c.Mail(from); err != nil {
c.Close()
continue
}
if err = c.Rcpt(to); err != nil {
c.Close()
continue
}
w, err := c.Data()
if err != nil {
c.Close()
continue
}
_, err = w.Write(signedData) // Use the DKIM signed message
if err != nil {
c.Close()
continue
}
err = w.Close()
if err != nil {
c.Close()
continue
}
c.Quit()
return nil
}
}
return fmt.Errorf("Failed to send email to %s", to)
}
In this updated sendMail function:
This implementation will add a DKIM signature to your outgoing emails, which will help improve deliverability and authenticity of your emails.
Remember to replace "path/to/your/private_key.pem" with the actual path to your DKIM private key, and update the Domain and Selector in dkimOptions to match your DKIM DNS record.
While this implementation provides a basic working SMTP server capable of receiving and sending emails, there are several important considerations for a production-ready server:
We hope you learned a lot by reading this post. To know more about sending emails, feel free to take a look at the GitHub repository of Ferdinand, and explore the code.
The above is the detailed content of Build Your Own SMTP Server in Go. For more information, please follow other related articles on the PHP Chinese website!
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