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.

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. You can find the example codes in EASendMail Installation Path\Samples_{Programming language/Developer Tool}\Oauth.* project.
If you used .NET framework 4.6.1 or later version, you can use Google.Apis.Auth.OAuth2 to request access token directly instead of using the codes in our sample 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.

Example

[C# - Get access token and user with Google.Apis.Auth.OAuth2]
// You can install Google.Apis.Auth.OAuth2 by NuGet
// Install-Package Google.Apis.Auth

using Google.Apis.Auth;
using Google.Apis.Auth.OAuth2;

var credential = await GoogleWebAuthorizationBroker.AuthorizeAsync(
        new ClientSecrets
        {
            ClientId = "put your client id here",
            ClientSecret = "put your client secret here"
        },
        new[] { "email", "profile", "https://mail.google.com/" },
        "user",
        CancellationToken.None
        );

var jwtPayload = GoogleJsonWebSignature.ValidateAsync(credential.Token.IdToken).Result;
var username = jwtPayload.Email;

// then you can get access token and user from the following properties
// credential.Token.AccessToken;
// username;

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.

[VB - Send Email using Gmail API + OAUTH Authentication]
    
Imports EASendMail

Sub SendMailWithXOAUTH2(userEmail As String, accessToken As String)

    Try
    ' set Gmail API address
        Dim oServer As SmtpServer = New SmtpServer("https://www.googleapis.com/upload/gmail/v1/users/me/messages/send?uploadType=media")

    ' set Gmail RESTFul API protocol 
        oServer.Protocol = ServerProtocol.GmailApi

    ' set SSL connection
        oServer.ConnectType = SmtpConnectType.ConnectSSLAuto
        
    ' use Gmail OAUTH 2.0 authentication
        oServer.AuthType = SmtpAuthType.XOAUTH2
    ' set user authentication
        oServer.User = userEmail
    ' use access token as password
        oServer.Password = accessToken

        Dim oMail As SmtpMail = New SmtpMail("TryIt")
        oMail.From = New MailAddress(userEmail)
        oMail.To.Add(New MailAddress("support@emailarchitect.net"))

        oMail.Subject = "test email sent from VB using Gmail OAUTH"
        oMail.TextBody = "test body"

        Dim oSmtp As SmtpClient = New SmtpClient
        oSmtp.SendMail(oServer, oMail)

        Console.WriteLine("The email has been submitted to server successfully!")

    Catch exp As Exception
        Console.WriteLine("Exception: {0}", exp.Message)
    End Try

End Sub
    




[C# - Send Email using Gmail API + OAUTH Authentication]

using System;
using EASendMail;

void SendMailWithXOAUTH2(string userEmail, string accessToken)
{
    try
    {
    // Gmail API server address
        SmtpServer oServer = new SmtpServer("https://www.googleapis.com/upload/gmail/v1/users/me/messages/send?uploadType=media");

    // set Gmail RESTFul API protocol 
        oServer.Protocol = ServerProtocol.GmailApi;

    // enable SSL connection
        oServer.ConnectType = SmtpConnectType.ConnectSSLAuto;
        
    // use Gmail OAUTH 2.0 authentication
        oServer.AuthType = SmtpAuthType.XOAUTH2;
    // set user authentication
        oServer.User = userEmail;
    // use access token as password
        oServer.Password = accessToken;

        SmtpMail oMail = new SmtpMail("TryIt");
    // Your gmail email address
        oMail.From = userEmail;
        oMail.To = "support@emailarchitect.net";

        oMail.Subject = "test email from gmail account with OAUTH 2";
        oMail.TextBody = "this is a test email sent from c# project with gmail.";

        Console.WriteLine("start to send email using OAUTH 2.0 ...");

        SmtpClient oSmtp = new SmtpClient();
        oSmtp.SendMail(oServer, oMail);

        Console.WriteLine("The email has been submitted to server successfully!");
    }
    catch (Exception ep)
    {
        Console.WriteLine("Exception: {0}", ep.Message);
    }
}





[C++/CLI - using Gmail API + OAUTH Authentication]

using namespace System;
using namespace EASendMail;

void SendMailWithXOAUTH2(String^ userEmail, String^ accessToken)
{
    try
    {
    // Gmail API server address
        SmtpServer ^oServer = gcnew SmtpServer("https://www.googleapis.com/upload/gmail/v1/users/me/messages/send?uploadType=media");

    // set Gmail RESTFul API protocol 
        oServer->Protocol = ServerProtocol::GmailApi;

    // set SSL connection
        oServer->ConnectType = SmtpConnectType::ConnectSSLAuto;

    // use Gmail OAUTH 2.0 authentication
        oServer->AuthType = SmtpAuthType::XOAUTH2;
    //set user authentication
        oServer->User = userEmail;
    // use access token as password
        oServer->Password = accessToken;

        SmtpMail ^oMail = gcnew SmtpMail("TryIt");
        oMail->From = gcnew MailAddress(userEmail);
        oMail->To->Add(gcnew MailAddress("support@emailarchitect.net"));

        oMail->Subject = "test email sent from C++/CLI using Gmail OAUTH";
        oMail->TextBody = "test body";

        SmtpClient ^oSmtp = gcnew SmtpClient();
        oSmtp->SendMail(oServer, oMail);

        Console::WriteLine("The email has been submitted to server successfully!");
    }
    catch (System::Exception ^exp)
    {
        Console::WriteLine("Exception: {0}", exp->Message);
    }
}

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 Tutorial

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

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

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