How to create and send MIME messages with attachments in Go using the net/smtp package?

To create and send MIME messages with attachments in Go using the net/smtp package, you can follow the steps below:

Import necessary packages:

package main

import (
	"net/smtp"
	"net/textproto"
	"mime/multipart"
	"mime/quotedprintable"
	"bytes"
	"io/ioutil"
)

Define your email message struct:

type EmailMessage struct {
	From      string
	To        []string
	Subject   string
	TextBody  string
	HTMLBody  string
	Attachment []Attachment
}

Define your attachment struct:

type Attachment struct {
	Filename string
	Content  []byte
}

Create a function to send the email:

func SendEmailMessage(smtpHost string, smtpPort int, smtpUsername string, smtpPassword string, message *EmailMessage) error {
	auth := smtp.PlainAuth("", smtpUsername, smtpPassword, smtpHost)

	// Convert the email message into MIME format
	mimeMessage := messageToMIME(message)

	// Connect to the SMTP server
	smtpAddr := fmt.Sprintf("%s:%d", smtpHost, smtpPort)
	client, err := smtp.Dial(smtpAddr)
	if err != nil {
		return err
	}
	defer client.Close()

	if err = client.Auth(auth); err != nil {
		return err
	}

	if err = client.Mail(message.From); err != nil {
		return err
	}
	
	// Loop through the recipients and add them to the email
	for _, recipient := range message.To {
		if err = client.Rcpt(recipient); err != nil {
			return err
		}
	}
	
	// Create a data writer to write the email data
	dataWriter, err := client.Data()
	if err != nil {
		return err
	}
	defer dataWriter.Close()

	// Write the MIME message to the data writer
	_, err = dataWriter.Write(mimeMessage.Bytes())
	if err != nil {
		return err
	}

	return nil
}

Create a function to convert the email message into MIME format:

func messageToMIME(message *EmailMessage) *bytes.Buffer {
	buffer := new(bytes.Buffer)
	writer := multipart.NewWriter(buffer)

	// Create a new text/plain part for the email body
	textPart, _ := writer.CreatePart(textproto.MIMEHeader{
		"Content-Type": []string{"text/plain; charset=utf-8"},
	})
	textPart.Write([]byte(message.TextBody))

	// Create a new text/html part for the email body if HTML body is present
	if message.HTMLBody != "" {
		htmlPart, _ := writer.CreatePart(textproto.MIMEHeader{
			"Content-Type": []string{"text/html; charset=utf-8"},
		})
		htmlPart.Write([]byte(message.HTMLBody))
	}

	// Add attachments to the email
	for _, attachment := range message.Attachment {
		part, _ := writer.CreateFormFile("attachment", attachment.Filename)
		part.Write(attachment.Content)
	}

	writer.Close()

	return buffer
}

Test sending an email:

func main() {
	// Define the email message
	message := &EmailMessage{
		From:     "[email protected]",
		To:       []string{"[email protected]"},
		Subject:  "Test Email with Attachment",
		TextBody: "This is a test email with an attachment.",
		HTMLBody: "<html><body><h1>This is a test email with an attachment.</h1></body></html>",
		Attachment: []Attachment{
			{
				Filename: "example.txt",
				Content:  []byte("This is the content of the attachment."),
			},
		},
	}
	
	// Call the send email function with your SMTP details
	err := SendEmailMessage("smtp.example.com", 587, "[email protected]", "password", message)
	if err != nil {
		fmt.Println("Error sending email:", err)
	} else {
		fmt.Println("Email sent successfully!")
	}
}

Ensure that you replace the placeholders smtp.example.com with your SMTP server's address, 587 with the SMTP server's port, [email protected] with the sender's email address, and [email protected] with the recipient's email address.