Using Gmail RESTFul API + OAUTH

Gmail supports SMTP + OAUTH, but the API (https://mail.google.com/) scope is restricted API which requests to have full access to the Gmail account. Restricted API is throttled before your project is authenticated in by Google.

Using less restricted API (https://www.googleapis.com/auth/gmail.send) scope to send email via Gmail server is recommended.

https://developers.google.com/gmail/oauth_overview?hl=en

Create your project in Google Developers Console

To use Gmail OAUTH in your application, you must create a project in Google Developers Console at first.

Create your project at https://console.developers.google.com/projectcreate.
Select your project -> APIs & Services -> Dashboard -> Credentials;
Credentials -> Create Credentials -> OAuth client ID -> Web application or Other (Desktop Application). It depends on your application type.
Input a name for your application, input your current ASP/ASP.NET URL at Authorized redirect URIs, for example: http://localhost/gmailoauth/default.aspx. (Desktop Application doesn't require this step)
Click "Create", you will get your client id and client secret:
Enable Gmail API in "Library" -> Search "Gmail", then click "Gmail API" and enable it.
You should set detail information for your project at "OAuth consent screen" -> "Edit App".
Finally add "https://mail.google.com/" and "../auth/gmail.send" scopes at "OAuth consent screen" -> "Edit App" -> "Scopes for Google API".

If you use Gmail RESTFul API to send email, please only use "../auth/gmail.send" scope to avoid your app throttled.

Scope Remarks

Using less restricted API (https://www.googleapis.com/auth/gmail.send) scope to send email via Gmail server is recommended.

Use client id and client secret to get access token

You can use client id and client secret to get the user email address and access token like this:

Your application uses a web browser/browser control to open Google OAUTH URL;
User inputs user and password in Google Web Authentication Page, and then Google returns access token back to your application;
Your application uses access token to access Gmail SMTP server/Gmail API. You can find the example codes in EASendMail Installation Path\Samples_{Programming language/Developer Tool}\Oauth.* project.

You should create your client id and client secret, don't use the client id in the sample project, it is only for test purpose. If you got "This app isn't verified" information, please click "advanced" -> Go to ... for test.

Use access token to send email via Gmail RESTFul API

After you get user email address and access token, you can use the following codes to send email using Gmail SASL XOAUTH2 mechanism.

Example

[Visual Basic 6.0, VC++, Delphi] To get the full samples of EASendMail, please refer to Samples section.

[VB6 - Send Email using Gmail API + OAUTH Authentication]

Const ConnectNormal = 0
Const ConnectSSLAuto = 1
Const ConnectSTARTTLS = 2
Const ConnectDirectSSL = 3
Const ConnectTryTLS = 4

Const AuthAuto = -1
Const AuthLogin = 0
Const AuthNtlm = 1
Const AuthCramMd5 = 2
Const AuthPlain = 3
Const AuthMsn = 4
Const AuthXoauth2 = 5

Private Sub SendEmail(email As String, access_token As String)
  Dim oSmtp As EASendMailObjLib.Mail
  Set oSmtp = New EASendMailObjLib.Mail
  oSmtp.LicenseCode = "TryIt"

  ' Set Gmail API adddress
  oSmtp.ServerAddr = "https://www.googleapis.com/upload/gmail/v1/users/me/messages/send?uploadType=media"

  ' Set Gmail API Protocol
  oSmtp.Protocol = 3;

  ' Enable SSL/TLS connection
  oSmtp.ConnectType = ConnectSSLAuto

  ' Gmail OAUTH/XOAUTH2 type
  oSmtp.AuthType = AuthXoauth2 
  oSmtp.UserName = email
  oSmtp.Password = access_token
  
  oSmtp.FromAddr = email
  oSmtp.AddRecipient "Support Team", "support@emailarchitect.net", 0
 
  oSmtp.BodyText = "Hello, this is a test...."
  If oSmtp.SendMail() = 0 Then
    MsgBox "Message delivered!"
  Else
    MsgBox oSmtp.GetLastErrDescription()
  End If

End Sub


[Visual C++ - Gmail API + OAUTH User Authentication]

#include "stdafx.h"
#include <tchar.h>
#include <Windows.h>

#include "EASendMailObj.tlh"
using namespace EASendMailObjLib;

const int ConnectNormal = 0;
const int ConnectSSLAuto = 1;
const int ConnectSTARTTLS = 2;
const int ConnectDirectSSL = 3;
const int ConnectTryTLS = 4;

const int AuthAuto = -1;
const int AuthLogin = 0;
const int AuthNtlm = 1;
const int AuthCramMd5 = 2;
const int AuthPlain = 3;
const int AuthMsn = 4;
const int AuthXoauth2 = 5;
 
void SendEmail(const TCHAR* lpszEmail, const TCHAR* lpszAccessToken)
{
  ::CoInitialize(NULL);
  IMailPtr oSmtp = NULL;
  oSmtp.CreateInstance(__uuidof(EASendMailObjLib::Mail));
  oSmtp->LicenseCode = _T("TryIt");
	
  // Set Gmail API adddress
  oSmtp->ServerAddr := _T("https://www.googleapis.com/upload/gmail/v1/users/me/messages/send?uploadType=media");

  // Set Gmail API Protocol
  oSmtp->Protocol := 3;
  
  // Enable SSL/TLS connection
  oSmtp->ConnectType = ConnectSSLAuto;

  // Gmail OAUTH type
  oSmtp->AuthType = AuthXoauth2; 
  oSmtp->UserName = lpszEmail;
  oSmtp->Password = lpszAccessToken;
  
  oSmtp->FromAddr = lpszEmail;
  oSmtp->AddRecipient(_T("Support Team"), 
                          _T("support@emailarchitect.net"), 0);
 
  oSmtp->BodyText = _T("Hello, this is a test....");
  if (oSmtp->SendMail() == 0)
    _tprintf(_T("Message delivered!"));
  else
    _tprintf((const TCHAR*)oSmtp->GetLastErrDescription());
}


[Delphi - Gmail API + OAUTH User Authentication]

const
    ConnectNormal = 0;
    ConnectSSLAuto = 1;
    ConnectSTARTTLS = 2;
    ConnectDirectSSL = 3;
    ConnectTryTLS = 4;

    AuthAuto = -1;
    AuthLogin = 0;
    AuthNtlm = 1;
    AuthCramMd5 = 2;
    AuthPlain = 3;
    AuthMsn = 4;
    AuthXoauth2 = 5;

procedure TForm1.SendEmail(email: string, accesstoken: string);
var
  oSmtp : TMail;
begin
  oSmtp := TMail.Create(Application);
  oSmtp.LicenseCode := 'TryIt';

  // Set Gmail API adddress
  oSmtp.ServerAddr := 'https://www.googleapis.com/upload/gmail/v1/users/me/messages/send?uploadType=media';

  // Set Gmail API Protocol
  oSmtp.Protocol := 3;
  
  // Enable SSL/TLS connection
  oSmtp.ConnectType := ConnectSSLAuto;

  // Gmail OAUTH type
  oSmtp.AuthType := AuthXoauth2; 
  oSmtp.UserName := email;
  oSmtp.Password := accesstoken;

  // Set sender email address
  oSmtp.FromAddr := email;

  // Add recipient email address
  oSmtp.AddRecipientEx('support@emailarchitect.net', 0);

  // Set email subject
  oSmtp.Subject := 'simple email from Delphi project';

  // Set email body
  oSmtp.BodyText := 'this is a test email sent from Delphi project, do not reply';
  
  ShowMessage('start to send email ...');

  if oSmtp.SendMail() = 0 then
    ShowMessage('email was sent successfully!')
  else
    ShowMessage('failed to send email with the following error: '
    + oSmtp.GetLastErrDescription());

end;

end.

Remarks

If your application is background service which doesn't support user interaction, please have a look at this topic: Use G Suite service account to do SMTP OAUTH.

If you don't want to use OAUTH 2.0, Gmail also supports traditional ESMTP authentication, but you need to enable Allowing less secure apps or Sign in using App Passwords.

Online Example

VB6 - Send Email using Google/Gmail OAuth 2.0 Authentication
VB6 - Send Email using Gmail/G Suite OAuth 2.0 in Background Service (Service Account)
VB6 - Send Email using Microsoft OAuth 2.0 (Modern Authentication) from Hotmail/Outlook Account
VB6 - Send Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 Account
VB6 - Send Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 in Background Service

Delphi - Send Email using Google/Gmail OAuth 2.0 Authentication
Delphi - Send Email using Gmail/G Suite OAuth 2.0 in Background Service (Service Account)
Delphi - Send Email using Microsoft OAuth 2.0 (Modern Authentication) from Hotmail/Outlook Account
Delphi - Send Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 Account
Delphi - Send Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 in Background Service

Visual C++ - Send Email using Google/Gmail OAuth 2.0 Authentication
Visual C++ - Send Email using Gmail/G Suite OAuth 2.0 in Background Service (Service Account)
Visual C++ - Send Email using Microsoft OAuth 2.0 (Modern Authentication) from Hotmail/Outlook Account
Visual C++ - Send Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 Account
Visual C++ - Send Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 in Background Service