How to write mbox-formatted files?

I have obtained the mail content and the necessary headers using Gmail API. I want to write those into mbox files. I could find Go packages and samples to read and parse mbox files. But how to create and write mbox file using Go?

The mbox file format (Wikipedia) is actually super-simple.

Each mail has a first line that starts "From ". Any first line in the email body that happens to start with "From " has either " " or ">" prepended. After each mail body, there's an extra blank line inserted. Typically, a mail header already has a "From ..." first line, so what you need to do is "iterate through each email, print it, scan the body to ensure all lines that start "From " have an escape, then finish each mail with a blank line".

Something like the following (will need adapting to how you represent emails):

package main

import (
	"fmt"
	"io"
	"os"
	"strings"
)

type Mail struct {
	Headers []string
	Body    []string
}

func (m *Mail) Save(w io.Writer) {
	for _, h := range m.Headers {
		fmt.Fprintln(w, h)
	}
	fmt.Println("")
	for _, b := range m.Body {
		if strings.HasPrefix(b, "From ") {
			fmt.Fprintln(w, ">", b)
		} else {
			fmt.Fprintln(w, b)
		}
	}
}

func WriteMbox(w io.Writer, mails []Mail) {
	for _, m := range mails {
		m.Save(w)
		fmt.Fprintln(w, "")
	}
}

func main() {
	m := Mail{Headers: []string{"From test", "Subject: Test"}, 
              Body: []string{"Mail body, totes, like"}}
	WriteMbox(os.Stdout, []Mail{m, m, m})
}