Send email with Go (net/smtp) over SMTP

# standard library — or: go get github.com/wneessen/go-mail (attachments, HTML, retries)

package main

import (
	"crypto/tls"
	"fmt"
	"net/smtp"
	"os"
	"strings"
)

func main() {
	host := getenv("SMTP_HOST", "smtp.queensmtp.com")
	port := getenv("SMTP_PORT", "587")
	user, pass := os.Getenv("SMTP_USER"), os.Getenv("SMTP_PASS")
	from, to := "hello@yourdomain.com", "customer@example.com"

	msg := strings.Join([]string{
		"From: Your App <" + from + ">",
		"To: " + to,
		"Subject: Your receipt",
		"MIME-Version: 1.0",
		"Content-Type: text/html; charset=UTF-8",
		"",
		"<h1>Thanks for your order</h1><p>#1042 is confirmed.</p>",
	}, "\r\n")

	auth := smtp.PlainAuth("", user, pass, host)
	tlsCfg := &tls.Config{ServerName: host}

	var c *smtp.Client
	var err error
	if port == "465" { // implicit TLS
		conn, e := tls.Dial("tcp", host+":"+port, tlsCfg)
		if e != nil { panic(e) }
		c, err = smtp.NewClient(conn, host)
	} else {           // STARTTLS
		c, err = smtp.Dial(host + ":" + port)
		if err == nil { err = c.StartTLS(tlsCfg) }
	}
	if err != nil { panic(err) }
	defer c.Quit()

	if err = c.Auth(auth); err != nil { panic(err) }
	if err = c.Mail(from); err != nil { panic(err) }
	if err = c.Rcpt(to); err != nil { panic(err) }
	w, err := c.Data()
	if err != nil { panic(err) }
	fmt.Fprint(w, msg)
	w.Close()
	fmt.Println("sent")
}

func getenv(k, d string) string { if v := os.Getenv(k); v != "" { return v }; return d }