Certificate verification error when connecting from Go

[muety]

When attempting to send mail from a Go application via an smtp4dev instance that runs with a self-signed certificate, I'm getting the following error:

tls: failed to verify certificate: x509: certificate relies on legacy Common Name field, use SANs instead

There used to be a flag that you could set to instruct Go to ignore this error (see here), but apparently it has been removed after Go 1.17.

Minimal code example:

package main

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

func main() {
	auth := smtp.PlainAuth("", "admin", "admin", "localhost.localdomain")
	err := smtp.SendMail("localhost.localdomain:2525", auth, "admin@example.org", []string{ "user@example.org" }, []byte("Just a test"))
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
}