Send a mail with a service account using Gmail API

[Vincent Jouglard]

Hi there,

I have been trying for quite some time to get over an error while trying to send an email using a service account, via the Gmail API.

I get:

googleapi: Error 400: Precondition check failed., failedPrecondition

Code is:

package gmailApi

import (

   "encoding/base64"

   "fmt"

   "net/mail"

   "strings"

    "golang.org/x/net/context"

   "google.golang.org/api/gmail/v1"

   "google.golang.org/api/option"

)

func SendEmailVerification() {

   srv, err := gmail.NewService(context.Background(), option.WithCredentialsFile("credentials.json"), option.WithScopes("https://mail.google.com/", "https://www.googleapis.com/auth/gmail.modify", "https://www.googleapis.com/auth/gmail.compose", "https://www.googleapis.com/auth/gmail.send"))

   if err != nil {

       fmt.Println(err) // BUG(Vincent): TEST

   }

   // New message for our gmail service to send

   var message gmail.Message

   // Compose the message

// @ symbol modified for publishing on golang-nuts

   messageStr := []byte(

       "From: vincent[at]gamifly.gg\r\n" +

           "To: jouglardv[at]gmail.com\r\n" +

           "Subject: My first Gmail API message\r\n\r\n" +

           "Message body goes here!")

    // Place messageStr into message.Raw in base64 encoded format

   message.Raw = base64.URLEncoding.EncodeToString(messageStr)

    if rslt, err := srv.Users.GetProfile("me").Do(); err != nil {

       fmt.Println(err) // BUG(Vincent): TEST

   } else {

       fmt.Println("111", rslt)

   }

    msg := ComposeMessage()

   _, err = srv.Users.Messages.Send("me", msg).Do()

   if err != nil {

       fmt.Println(err) // BUG(Vincent): TEST

   } else {

       fmt.Println("OKOKOK")

   }

}

func ComposeMessage() *gmail.Message {

   from := mail.Address{"Vincent", "vincent[at]gamifly.gg"} // @ symbol modified for publishing on golang-nuts

   to := mail.Address{"Vincent", "jouglardv[at]gmail.com"} // @ symbol modified for publishing on golang-nuts

    header := make(map[string]string)

   header["From"] = from.String()

   header["To"] = to.String()

   header["Subject"] = encodeRFC2047("Hello, Gmail!")

   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" + "Message"

    return &gmail.Message{

       Raw: base64.RawURLEncoding.EncodeToString([]byte(msg)),

   }

}

func encodeRFC2047(s string) string {

   // use mail's rfc2047 to encode any string

   addr := mail.Address{s, ""}

   return strings.Trim(addr.String(), " <>")

}

 

vincent[at]gamifly.gg is the GSuite user that is being impersonated.

jouglardv[at]gmail.com is a personnal address for testing purpose (plz don't spam <3 )

The service account has domain delegation activated, scopes that are passed are in API Controls > Domain-Wide Delegation: 

https://www.googleapis.com/auth/gmail.send, https://mail.google.com/, https://www.googleapis.com/auth/gmail.modify, https://www.googleapis.com/auth/gmail.compose

Is there anything wrong that I may be doing ?

Thanks guys

[Vincent Jouglard]

Hi John,

Thanks for your message.
The main difference with my implementation is that I use a service account which is authorized to send messages on my behalf...and I think that this is where lies the problem.

As i understand OAuth and the implementation from your exemple, a token is generated and could (will) need to be refreshed, requiring a human interaction (copy past a token from a UrL). Using a service account allows a full server-to-server flow.
Please correct me if I am mistaken!

[Vincent Jouglard]

Hi John,

Thanks for you answer, which led me to the correct configuration.

It was all about the "subject" missing from my way of implementing the Service ; as the function gmail.New() is deprecated, gmail.NewService() is to be used instead...but it does not implement - yet - the "subject" prop.

A discussion is still opened to fix this lack of code: https://github.com/googleapis/google-api-go-client/issues/379

This describes exactly the code you sent me (btw, I don't know I received your answers by mail but they are not displayed here on groups.google.com)...which is working like a charm.

Thanks a lot again,