send email with attachment in golang

sendemail.go

package smtpserver

import (
	"bytes"
	"crypto/tls"
	"encoding/base64"
	"fmt"
	"mime/multipart"
	"net/smtp"
)

func SendEmail(config *configs.Configs, to, subject, body string, attachments map[string][]byte) error {
	from := config.Env.GetKey("smtp.mailfromaddress")

	// Setup headers
	headers := make(map[string]string)
	headers["From"] = from
	headers["To"] = to
	headers["Subject"] = subject

	// Setup message
	var message bytes.Buffer
	for key, value := range headers {
		message.WriteString(fmt.Sprintf("%s: %s\n", key, value))
	}

	// Create a multipart writer for the email body
	multipartWriter := multipart.NewWriter(&message)

	message.WriteString("MIME-Version: 1.0\r\n")
	message.WriteString("Content-Type: multipart/mixed; boundary=" + multipartWriter.Boundary() + "\r\n\r\n")

	// Add the text body part
	message.WriteString("--" + multipartWriter.Boundary() + "\r\n")
	message.WriteString("Content-Type: text/plain; charset=utf-8\r\n")
	message.WriteString("Content-Transfer-Encoding: quoted-printable\r\n\r\n")
	message.WriteString(body + "\r\n")

	// Attach each file to the email
	for name, attachment := range attachments {
		message.WriteString("--" + multipartWriter.Boundary() + "\r\n")
		message.WriteString("Content-Type: application/octet-stream\r\n")
		message.WriteString("Content-Transfer-Encoding: base64\r\n")
		message.WriteString("Content-Disposition: attachment; filename=\"" + name + "\"\r\n\r\n")

		// Convert data to base64 and write to the message buffer
		encoder := base64.NewEncoder(base64.StdEncoding, &message)
		encoder.Write(attachment)
		encoder.Close()

		message.WriteString("\r\n")
	}

	multipartWriter.Close()

	host := config.Env.GetKey("smtp.host")
	servername := fmt.Sprintf("%s:%s", host, config.Env.GetKey("smtp.port"))

	// Connect to the SMTP Server
	auth := smtp.PlainAuth("", config.Env.GetKey("smtp.username"), config.Env.GetKey("smtp.password"), host)

	// TLS config
	tlsconfig := &tls.Config{
		InsecureSkipVerify: true,
		ServerName:         host,
	}

	// Here is the key, you need to call tls.Dial instead of smtp.Dial
	// for smtp servers running on 465 that require an ssl connection
	// from the very beginning (no starttls)
	c, err := smtp.Dial(servername)
	if err != nil {
		return err
	}

	if err = c.StartTLS(tlsconfig); err != nil {
		return err
	}

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

	err = smtp.SendMail(servername, auth, from, []string{to}, message.Bytes())
	if err != nil {
		return err
	}

	return nil
}