How to send email through Gmail Go SDK?

I'm trying to send a new email through the gmail package . However the Message type which is required by the send method is poorly documented. Most of the fields seem used to actually parse/read emails. The only field which makes sense (at some degree) for the send method is Payload of type MessagePart though I can't figure it out how to generate the MessagePartBody as it seems to be a kind of mime type. Below is the code I have so far.

func (em *Email) SendMessage(cl *Client) error {
config.ClientId = cl.Username
config.ClientSecret = cl.Password
t := &oauth.Transport{
Config:    config,
Transport: http.DefaultTransport,
}
var tk oauth.Token
err := json.Unmarshal([]byte(cl.Meta), &tk)
t.Token = &tk
if err != nil {
log.Errorf("meta %v, err %v", cl.Meta, err)
return err
}
gmailService, err := gmail.New(t.Client())
if err != nil {
log.Error(err)
return err
}
p := gmail.MessagePart{}
p.Headers = append(p.Headers, &gmail.MessagePartHeader{
Name:  "From",
Value: em.FromEmail,
})
p.Headers = append(p.Headers, &gmail.MessagePartHeader{
Name:  "To",
Value: em.ToEmail,
})
p.Headers = append(p.Headers, &gmail.MessagePartHeader{
Name:  "Subject",
Value: em.Subject,
})
emsg := base64.StdEncoding.EncodeToString(em.Message)
log.Info(emsg)
msg := gmail.Message{
Payload: &p,
Raw:     "",
}
_, err = gmailService.Users.Messages.Send("me", &msg).Do()
if err != nil {
log.Error(err)
return err
}
return err
}

The "REST" API is even more confusing. It requires an uploadType param (WTF to upload) and a raw field which I guess is the raw message which requires a format provided by messages.get. Why would you send a message from your inbox which literally would be a 'resend' as your are on the receipt list ? Am I the only one who thinks this API(or at least the documentation) is just crap ?

It was a bit tricky but here is how you can send emails through the GMAIL API

import(
"code.google.com/p/goauth2/oauth"
"code.google.com/p/google-api-go-client/gmail/v1"
log "github.com/golang/glog"
"encoding/base64"
"encoding/json"
"net/mail"
"strings"
)
type Email struct {
FromName, FromEmail, ToName, ToEmail, Subject string
Message                                       string
}
func (em *Email) SendMessage(cl *Client) error {
config.ClientId = cl.Username //oauth clientID
config.ClientSecret = cl.Password  //oauth client secret 
t := &oauth.Transport{
Config:    config,
Transport: http.DefaultTransport,
}
var tk oauth.Token
err := json.Unmarshal([]byte(cl.Meta), &tk)
t.Token = &tk
if err != nil {
log.Errorf("meta %v, err %v", cl.Meta, err)
return err
}
gmailService, err := gmail.New(t.Client())
if err != nil {
log.Error(err)
return err
}
from := mail.Address{em.FromName, em.FromEmail}
to := mail.Address{em.ToName, em.ToEmail}
header := make(map[string]string)
header["From"] = from.String()
header["To"] = to.String()
header["Subject"] = encodeRFC2047(em.Subject)
header["MIME-Version"] = "1.0"
header["Content-Type"] = "text/html; charset=\"utf-8\""
header["Content-Transfer-Encoding"] = "base64"
var msg string
for k, v := range header {
msg += fmt.Sprintf("%s: %s\r\n", k, v)
}
msg += "\r\n" + em.Message
gmsg := gmail.Message{
Raw: encodeWeb64String([]byte(msg)),
}
_, err = gmailService.Users.Messages.Send("me", &gmsg).Do()
if err != nil {
log.Errorf("em %v, err %v", gmsg, err)
return err
}
return err
}
func encodeRFC2047(s string) string {
// use mail's rfc2047 to encode any string
addr := mail.Address{s, ""}
return strings.Trim(addr.String(), " <>")
}
func encodeWeb64String(b []byte) string {
s := base64.URLEncoding.EncodeToString(b)
var i = len(s) - 1
for s[i] == '=' {
i--
}
return s[0 : i+1]
}

Similar to @hey 's answer, but I tidied it up, and allowed the email to put newlines in the email body through \n and show up correctly on the email client. Also, @hey is not using the new supported Gmail API. Here is the final code:

import (
"encoding/base64"
"golang.org/x/net/context"
"golang.org/x/oauth2"
"golang.org/x/oauth2/google"
"google.golang.org/api/gmail/v1"
"encoding/json"
"net/mail"
)
type Email struct {
FromName  string
FromEmail string
ToName    string
ToEmail   string
Subject   string
Message   string
}
func (em *Email) sendMailFromEmail() error {
b, err := ioutil.ReadFile("credentials.json")
if err != nil {
log.Fatalf("Unable to read client secret file: %v", err)
}
// If modifying these scopes, delete your previously saved token.json.
config, err := google.ConfigFromJSON(b, gmail.GmailSendScope)
if err != nil {
log.Fatalf("Unable to parse client secret file to config: %v", err)
}
cl := getClientMail(config)
gmailService, err := gmail.New(cl)
if err != nil {
log.Fatalf("Unable to retrieve Gmail client: %v", err)
}
from := mail.Address{em.FromName, em.FromEmail}
to := mail.Address{em.ToName, em.ToEmail}
header := make(map[string]string)
header["From"] = from.String()
header["To"] = to.String()
header["Subject"] = em.Subject
header["MIME-Version"] = "1.0"
header["Content-Type"] = "text/plain; charset=\"utf-8\""
header["Content-Transfer-Encoding"] = "base64"
var msg string
for k, v := range header {
msg += fmt.Sprintf("%s: %s\r\n", k, v)
}
msg += "\r\n" + em.Message
gmsg := gmail.Message{
Raw: base64.RawURLEncoding.EncodeToString([]byte(msg)),
}
_, err = gmailService.Users.Messages.Send("me", &gmsg).Do()
if err != nil {
log.Printf("em %v, err %v", gmsg, err)
return err
}
return err
}

I did not include the following functions: getClient, getTokenFromWeb, tokenFromFile, and saveToken. You can find them, and learn how to enable the Gmail API through this tutorial by Google.