Go

1Send with net/smtp

package main

import (
	"fmt"
	"net/smtp"
)

func main() {
	host := "smtp.mailclutch.com"
	auth := smtp.PlainAuth("", "mc_0000000000000000", "pw_0000000000000000", host)

	msg := []byte("To: anyone@example.com\r\n" +
		"From: staging@example.com\r\n" +
		"Subject: Sandbox check\r\n" +
		"\r\n" +
		"Hello from Go.\r\n")

	err := smtp.SendMail(
		fmt.Sprintf("%s:%d", host, 587),
		auth,
		"staging@example.com",
		[]string{"anyone@example.com"},
		msg,
	)
	if err != nil {
		panic(err)
	}
}

smtp.SendMail upgrades the connection with STARTTLS when the server offers it, which ours does on every port but 465.

2Or with a library

For attachments and HTML parts, a library saves you writing MIME by hand:

d := gomail.NewDialer("smtp.mailclutch.com", 587, "mc_0000000000000000", "pw_0000000000000000")

m := gomail.NewMessage()
m.SetHeader("From", "staging@example.com")
m.SetHeader("To", "anyone@example.com")
m.SetHeader("Subject", "Sandbox check")
m.SetBody("text/html", "<h1>Hello from Go</h1>")

if err := d.DialAndSend(m); err != nil {
	panic(err)
}

Watch it land

Open the sandbox and the message is there — rendered HTML, plain text, raw source, attachments, and the SPF, DKIM and DMARC verdicts. It updates live, so you can leave it open next to your terminal.

Nothing is ever delivered onward, whatever address you send to; claim the inbox with an account and it stays.

What usually goes wrong

• Headers are part of the body

  net/smtp sends exactly the bytes you hand it. The To, From and Subject lines have to be in the message, separated from the body by a blank line, with CRLF endings.

• Port 465

  Implicit TLS is not something smtp.SendMail does. Use tls.Dial plus smtp.NewClient for 465, or stay on 587.

Other stacks

The same four values in another dialect. Every guide below is filled in with the sandbox pair at the top of this page.