Golang - Pipe to external exec

I'm trying to use Go to pipe an email message to the sendmail binary. The following code works providing I don't uncomment the call to Wait(). If it's uncommented, the program hangs. Reading the documentation for wait, I get the impression that it should be called so I'm concluding there's an error in my example code. Any suggestions for what it might be?

package main

import (
        "io"
        "os"
        "os/exec"
        "fmt"
)

func main() {
        msg := "From: source@email.address\n"
        msg += "To: recipient@email.address\n"
        msg += "Subject: Testing\n\n"
        msg += "Hello World!\n"
        sendmail := exec.Command("/usr/sbin/sendmail", "-t")
        stdin, err := sendmail.StdinPipe()
        if err != nil {
                panic(err)
        }
        sendmail.Stdout = os.Stdout
        sendmail.Stderr = os.Stderr
        err = sendmail.Start()
        if err != nil {
                panic(err)
        }
        io.WriteString(stdin, msg)
        //err = sendmail.Wait()
        //if err != nil {
        //      panic(err)
        //}
}

What is (probably) happening is that sendmail is waiting for the input to finish.

Either close the pipe after having written or (depending on sendmail versions and stuff) try finishing sending through ".\n" as the very last piece of the message (some versions will accept that in addition to the input being closed).

NEVER NEVER NEVER use the "-t" command-line option here. It will mis-deliver mail, like sending messages from a mailing list back to the mailing list.