Golang SSL SMTP Example

ssl_smtp_example.go

package main

import (
    "fmt"
	  "log"
    "net"
    "net/mail"
	  "net/smtp"
    "crypto/tls"
)

// SSL/TLS Email Example

func main() {

    from := mail.Address{"", "username@example.tld"}
    to   := mail.Address{"", "username@anotherexample.tld"}
    subj := "This is the email subject"
    body := "This is an example body.\n With two lines."

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

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

    // Connect to the SMTP Server
    servername := "smtp.example.tld:465"

    host, _, _ := net.SplitHostPort(servername)

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

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

    // Here is the key, you need to call tls.Dial instead of smtp.Dial
    // for smtp servers running on 465 that require an ssl connection
    // from the very beginning (no starttls)
    conn, err := tls.Dial("tcp", servername, tlsconfig)
    if err != nil {
        log.Panic(err)
    }

    c, err := smtp.NewClient(conn, host)
    if err != nil {
        log.Panic(err)
    }

    // Auth
    if err = c.Auth(auth); err != nil {
        log.Panic(err)
    }

    // To && From
    if err = c.Mail(from.Address); err != nil {
        log.Panic(err)
    }

    if err = c.Rcpt(to.Address); err != nil {
        log.Panic(err)
    }

    // Data
    w, err := c.Data()
    if err != nil {
        log.Panic(err)
    }

    _, err = w.Write([]byte(message))
    if err != nil {
        log.Panic(err)
    }

    err = w.Close()
    if err != nil {
        log.Panic(err)
    }

    c.Quit()

}

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

Should we make the protection for SMTP-header-injection more explicit? like:
https://www.geeksforgeeks.org/what-is-smtp-header-injection/
https://github.com/golang/go/blob/87ec2c959c73e62bfae230ef7efca11ec2a90804/src/net/smtp/smtp.go#L429

// Setup message
message := ""
for k, v := range headers {
	line := fmt.Sprintf("%s: %s", k, v)
	// security assurance: RFC 5321
	if err := validateLine(line); err != nil {
		return err
	}
	message += line + "\r\n"
}
message += "\r\n" + body

// validateLine checks to see if a line has CR or LF as per RFC 5321.
func validateLine(line string) error {
	if strings.ContainsAny(line, "\n\r") {
		return errors.New("[mailer] smtp: A line must not contain CR or LF")
	}
	return nil
}

@gjedeer

To the people getting "first record does not look like a TLS handshake" on port 587: use port 465 instead, if your provider supports it.

Your suggestion worked for me! Thanks bro! Also thanks to @chrisgillis