Send mail using Go stdlib

sendmail.go

package main

import (
	"fmt"
	"bytes"
	"crypto/tls"
	"encoding/base64"
	"fmt"
	"net/mail"
	"net/smtp"
	"text/template"
)

type Sendmail interface {
	Context(context string) Sendmail
	ParseBody(tpl string, data map[string]string) Sendmail
	ParseSubject(tpl string, data map[string]string) Sendmail
	SendTo(email string) error
}

type SMTP struct {
	user    string
	secret  string
	sender  string
	from    string
	server  string
	port    string
	to      string
	context string
	body    string
	subject string
}

type Config struct {
	SMTPUser   string `json:"SMTP_USER" envconfig:"SMTP_USER"`
	SMTPSecret string `json:"SMTP_SECRET" envconfig:"SMTP_SECRET"`
	SMTPSender string `json:"SMTP_SENDER" envconfig:"SMTP_SENDER"`
	SMTPFrom   string `json:"SMTP_FROM" envconfig:"SMTP_FROM"`
	SMTPServer string `json:"SMTP_SERVER" envconfig:"SMTP_SERVER"`
	SMTPPort   string `json:"SMTP_PORT" envconfig:"SMTP_PORT"`
}

func NewSendmail(config *Config) interfaces.Sendmail {
	user := config.SMTPUser
	secret := config.SMTPSecret
	sender := config.SMTPSender
	from := config.SMTPFrom
	server := config.SMTPServer
	port := config.SMTPPort

	return &SMTP{
		user:   user,
		secret: secret,
		sender: sender,
		from:   from,
		server: server,
		port:   port,
	}
}

func (e *SMTP) ParseBody(tpl string, data map[string]string) interfaces.Sendmail {
	e.body = e.parseTemplate(tpl, data)
	return e
}

func (e *SMTP) ParseSubject(tpl string, data map[string]string) interfaces.Sendmail {
	e.subject = e.parseTemplate(tpl, data)
	return e
}

func (e *SMTP) parseTemplate(tpl string, data map[string]string) string {
	t := template.Must(template.New("tpl").Parse(tpl))
	buf := new(bytes.Buffer)
	t.Execute(buf, data)
	return buf.String()
}

func (e *SMTP) SendTo(email string) error {
	e.to = email
	return e.sendHTML()
}

func (e *SMTP) Context(context string) interfaces.Sendmail {
	e.context = context
	return e
}

func (e *SMTP) sendHTML() error {
	message := e.writeHTMLEmail(e.to, e.subject, e.body)
	return e.send(e.to, e.subject, message)
}

func (e *SMTP) send(dest string, subject, bodyMessage string) error {
	tlsconfig := &tls.Config{
		InsecureSkipVerify: true,
		ServerName:         e.server,
	}
	servername := fmt.Sprintf("%s:%s", e.server, e.port)

	conn, err := tls.Dial("tcp", servername, tlsconfig)
	if err != nil {
		return err
	}

	c, err := smtp.NewClient(conn, e.server)
	if err != nil {
		return err
	}
	defer c.Quit()

	auth := smtp.PlainAuth("", e.user, e.secret, e.server)
	if err = c.Auth(auth); err != nil {
		return err
	}

	if err = c.Mail(e.sender); err != nil {
		return err
	}

	if err = c.Rcpt(dest); err != nil {
		return err
	}

	w, err := c.Data()
	if err != nil {
		return err
	}

	_, err = w.Write([]byte(bodyMessage))
	if err != nil {
		return err
	}

	err = w.Close()
	if err != nil {
		return err
	}

	return nil
}

func (e *SMTP) writeHTMLEmail(to, subject, bodyMessage string) string {
	return e.writeEmail(to, "text/html", subject, bodyMessage)
}

func (e *SMTP) writeEmail(to, contentType, subject, bodyMessage string) string {
	from := mail.Address{Name: e.from, Address: e.sender}
	header := make(map[string]string)
	header["From"] = from.String()
	header["To"] = to
	header["Subject"] = subject
	header["MIME-Version"] = "1.0"
	header["Content-Type"] = fmt.Sprintf("%s; charset=\"utf-8\"", contentType)
	header["Content-Transfer-Encoding"] = "base64"

	var message string
	for key, value := range header {
		message += fmt.Sprintf("%s: %s\r\n", key, value)
	}
	message += "\r\n" + base64.StdEncoding.EncodeToString([]byte(bodyMessage))
	return message
}

type Template struct {
	Subject string
	Body string
}

type EmailData struct {
	FullName string
	ProductName string
}

func main() {
	config: &Config{
		SMTPUser: "apikey",
		SMTPSecret: "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",
		SMTPSender: "no-reply@domain.com"
		SMTPFrom: "Minha Empresa",
		SMTPServer: "smtp.sendgrid.net",
		SMTPPort: 465,
	}
	data := map[string]string{
		"fullName": "Meu usuário querido",
		"productName": "Meu incrível produto",
	}
	tpl := Template{
		Subject: "Bem-vindo {{ .FullName }}",
		Body: "<p>Parabéns {{ .FullName }}! Você acaba de comprar {{ .ProductName }}</p>",
	}
	data := map[string]string{
		"FullName": "Henrique",
		"ProductName": "Notebook",
	}
	sendmail := NewSendmail(config)
	err := sendmail.
		ParseSubject(tpl.Subject, data).
		ParseBody(tpl.Body, data).
		SendTo("cliente@gmail.com")
	fmt.Println("Error?", err)
}