Send email from Go with net/smtp

Before you start

1. Sign in and verify the domain you send from. Viesti accepts a message only when its From address is on a domain verified in your workspace and the envelope sender (MAIL FROM), which most libraries take from the From address, is on the same domain. DNS setup lists the records.
2. Create an API key on the Credentials page of the dashboard. It is shown once. The key is the SMTP password; the username is apikey.

Set the environment

export VIESTI_API_KEY="mr_your_api_key"
export MAIL_FROM="[email protected]"

SendMail passes the sender to the SMTP MAIL FROM command as is, so use a bare address without a display name.

Send an email

package main

import (
	"log"
	"net/smtp"
	"os"
)

func main() {
	const host = "smtp.viesti.email"
	from := os.Getenv("MAIL_FROM") // bare address on your verified domain
	to := []string{"[email protected]"}

	msg := []byte("From: " + from + "\r\n" +
		"To: [email protected]\r\n" +
		"Subject: Your receipt\r\n" +
		"MIME-Version: 1.0\r\n" +
		"Content-Type: text/plain; charset=utf-8\r\n" +
		"\r\n" +
		"Thanks for your order.\r\n")

	auth := smtp.PlainAuth("", "apikey", os.Getenv("VIESTI_API_KEY"), host)
	if err := smtp.SendMail(host+":587", auth, from, to, msg); err != nil {
		log.Fatal(err)
	}
	log.Print("accepted")
}

Viesti adds Date and Message-ID headers when a message has none. Tested with Go 1.26.

Test it

go run .

It logs accepted when Viesti takes the message. Check Activity in the dashboard.

Port 465

SendMail only speaks STARTTLS. On 465 TLS starts before the SMTP greeting, so dial with crypto/tls and drive the client yourself:

package main

import (
	"crypto/tls"
	"log"
	"net/smtp"
	"os"
)

func main() {
	const host = "smtp.viesti.email"
	from := os.Getenv("MAIL_FROM") // bare address on your verified domain
	to := []string{"[email protected]"}

	msg := []byte("From: " + from + "\r\n" +
		"To: [email protected]\r\n" +
		"Subject: Your receipt\r\n" +
		"MIME-Version: 1.0\r\n" +
		"Content-Type: text/plain; charset=utf-8\r\n" +
		"\r\n" +
		"Thanks for your order.\r\n")

	if err := sendImplicitTLS(host, from, to, msg); err != nil {
		log.Fatal(err)
	}
	log.Print("accepted")
}

// sendImplicitTLS submits msg on port 465, where TLS starts before the
// SMTP greeting; smtp.SendMail only speaks STARTTLS.
func sendImplicitTLS(host, from string, to []string, msg []byte) error {
	conn, err := tls.Dial("tcp", host+":465", &tls.Config{ServerName: host})
	if err != nil {
		return err
	}
	c, err := smtp.NewClient(conn, host)
	if err != nil {
		return err
	}
	defer c.Close()
	if err = c.Auth(smtp.PlainAuth("", "apikey", os.Getenv("VIESTI_API_KEY"), host)); err != nil {
		return err
	}
	if err = c.Mail(from); err != nil {
		return err
	}
	for _, addr := range to {
		if err = c.Rcpt(addr); err != nil {
			return err
		}
	}
	w, err := c.Data()
	if err != nil {
		return err
	}
	if _, err = w.Write(msg); err != nil {
		return err
	}
	if err = w.Close(); err != nil {
		return err
	}
	return c.Quit()
}

Why net/smtp

The package is frozen: its documentation says it accepts no new features. It still covers submission: STARTTLS when the server offers it, PLAIN authentication that is only sent over TLS or to localhost, and the SMTP commands. What it lacks is a message builder and timeouts. For HTML with a text alternative or attachments, build the body with mime/multipart or a third-party package; the connection code stays the same. For a deadline, open the connection yourself with a net.Dialer that sets Timeout (for port 465, tls.DialWithDialer) and hand it to smtp.NewClient, as the port 465 example hands over its connection.

Common errors

Replies from Viesti

Whatever the library, its error includes the server's reply. These are the ones caused by your setup or account:

A 250 reply means Viesti accepted the message. Content screening runs before relay; mail it flags is held for review and shows as quarantined on the Activity page.

Next steps

• DNS setup: DKIM, SPF, DMARC and return-path records for your domain.
• SMTP reference: ports, TLS and authentication in detail.
• HTTP API: send the same message as JSON over HTTPS.
• Limits and Bounces: quotas, message size and suppressions.

Package behavior is from the net/smtp documentation. Checked 26 September 2026.