Setting up standard Go net/smtp with Office 365 fails with "Error tls: first record does not look like a TLS handshake"

I'm trying to create a simple Go emailing service using the default Go packages net/smtp - I know there's gomailer, but i'd like to use the standard library

I need help with configuring the tls/server setting to work with Office365

I believe that I have the correct host:

smtp.office365.com:587

From copying the documentation for smtp that Microsoft provide, however, I get the following error in my console when running the below code:

Error: tls: first record does not look like a TLS handshake panic: runtime error: invalid memory address or nil pointer dereference

package main

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

func main() {

from := mail.Address{"", "example@example.com"}
to := mail.Address{"", "example@example.com"}
subject := "My test subject"
body := "Test email body"

// Setup email headers
headers := make(map[string]string)
headers["From"] = from.String()
headers["To"] = to.String()
headers["Subject"] = subject

message := ""
for k, v := range headers {
    message += fmt.Sprintf("%s: %s\r\n", k, v)
}
message += "\r\n" + body

servername := "smtp.office365.com:587"
host, _, _ := net.SplitHostPort(servername)

auth := smtp.PlainAuth("", "example@example.com", "password", host)

tlsconfig := &tls.Config{
    InsecureSkipVerify: true,
    ServerName:         host,
}

conn, err := tls.Dial("tcp", "smtp.office365.com:587", tlsconfig)
if err != nil {
    fmt.Println("tls.Dial Error: %s", err)
}

c, err := smtp.NewClient(conn, host)
if err != nil {
    fmt.Println("smtp.NewClient Error: %s", err)
}

if err = c.Auth(auth); err != nil {
    fmt.Println("c.Auth Error: %s", err)
}

if err = c.Mail(from.Address); err != nil {
    fmt.Println("c.Mail Error: %s", err)
}

if err = c.Rcpt(to.Address); err != nil {
    fmt.Println("c.Rcpt Error: %s", err)
}

w, err := c.Data()
if err != nil {
    fmt.Println("c.Data Error: %s", err)
}

_, err = w.Write([]byte(message))
if err != nil {
    fmt.Println("Error: %s", err)
}

err = w.Close()
if err != nil {
    fmt.Println("reader Error: %s", err)
}

c.Quit()
}

Any examples of an O365 client will be appreciated, or anything that anyone can spot that seems suspect will be great

Thanks