How to send an email with Golang using SMTP

In this article, I'll show you how to send an email with Golang using SMTP.

We'll be using the SMTP to send the email. You can use any SMTP service but here I'll use the Mailtrap. Mailtrap is a testing tool and is not designed to deliver emails to real addresses.

Here are some of the SMTP providers you can consider:

MailGun
SendGrid
Postmark
Sendinblue

Please refer to the corresponding documentation to understand the setup instructions.

To send an email using SMTP, you must collect the below information from your SMTP provider:

Host
Port
Username
Password

Create a new folder called send-email.

mkdir send-email

cd send-email

touch main.go

Open the main.go and import the necessary packages.

package main

import (
	"fmt"
	"net/smtp"
)

Let's declare the variables to hold the SMTP configuration.

func main() {
    // SMTP configuration
    username := "<smtp username>"
    password := "<smtp password>"
    host := "smtp.mailtrap.io"
    port := "25"
}

Replace the host and port values if you're using a different SMTP provider.

Add the variables to hold the subject, body, from, to.

// Subject and body
subject := "Simple HTML Email"
body := "Here is a simple plain text."

// Sender and receiver
from := "from@example.com"
to := []string{
  "to@example.com",
}

Now, let's build the message to be sent.

We'll append the from, to, subject, body to create the message. The message lines are separated with CRLF characters.

// Build the message
message := fmt.Sprintf("From: %s\r\n", from)
message += fmt.Sprintf("To: %s\r\n", to)
message += fmt.Sprintf("Subject: %s\r\n", subject)
message += fmt.Sprintf("\r\n%s\r\n", body)

Now call the PlainAuth method to start the authentication with the SMTP server. PlainAuth returns an Auth that implements the PLAIN authentication mechanism

// Authentication.
auth := smtp.PlainAuth("", username, password, host)

Now send the email using SendMail method. The method returns an error if sending an email is not successful. Make sure you catch the error properly.

// Send email
err := smtp.SendMail(host+":"+port, auth, from, to, []byte(message))
if err != nil {
  fmt.Println(err)
  return
}

fmt.Println("Email sent successfully.")

Here is the complete working code.

package main

import (
	"fmt"
	"net/smtp"
)

func main() {
  // SMTP configuration
  username := "<SMTP username>"
  password := "<SMTP password>"
  host := "<SMTP host>"
  port := "25"

  // Subject and body
  subject := "Simple HTML Email"
  body := "Here is a simple plain text."

  // Sender and receiver
  from := "from@example.com"
  to := []string{
    "to@example.com",
  }

  // Build the message
  message := fmt.Sprintf("From: %s\r\n", from)
  message += fmt.Sprintf("To: %s\r\n", to)
  message += fmt.Sprintf("Subject: %s\r\n", subject)
  message += fmt.Sprintf("\r\n%s\r\n", body)

  // Authentication.
  auth := smtp.PlainAuth("", username, password, host)

  // Send email
  err := smtp.SendMail(host+":"+port, auth, from, to, []byte(message))
  if err != nil {
    fmt.Println(err)
    return
  }

  fmt.Println("Email sent successfully.")
}