C#/ASP.NET/ASP MVC - Send email using Google/Gmail OAuth 2.0 authentication

By default, you need to enable ” Allowing less secure apps” in Gmail, then you can send email with user/password SMTP authentication.

However Google will disable traditional user authentication in the future, switching to Google OAuth is strongly recommended now.


Before you can use the following codes, please download EASendMail SMTP Component and install it on your machine at first. Full sample proejcts are included in this installer.

Install from NuGet

You can also install the run-time assembly by NuGet. Run the following command in the NuGet Package Manager Console:

Install-Package EASendMail


If you install it by NuGet, no sample projects are installed, only .NET assembly is installed.

Add reference

To use EASendMail SMTP Component in your project, the first step is Add reference of EASendMail to your project. Please create or open your project with Visual Studio, then go to menu -> Project -> Add Reference -> .NET -> Browse..., and select Installation Path\Lib\net[version]\EASendMail.dll from your disk, click Open -> OK, the reference of EASendMail will be added to your project, and you can start to use it to send email in your project.

add reference in c#/vb.net/c++/cli/clr

.NET assembly

Because EASendMail has separate builds for .Net Framework, please refer to the following table and choose the correct dll.

Separate builds of run-time assembly for .NET Framework 1.1, 2.0, 3.5, 4.0, 4.5, 4.6.1, .NET Core 3.1, .NET 5.0, .NET Standard 2.0 and .NET Compact Framework 2.0, 3.5.

File .NET Framework Version
Lib\net20\EASendMail.dll Built with .NET Framework 2.0
It requires .NET Framework 2.0, 3.5 or later version.
Lib\net40\EASendMail.dll Built with .NET Framework 4.0
It requires .NET Framework 4.0 or later version.
Lib\net45\EASendMail.dll Built with .NET Framework 4.5
It requires .NET Framework 4.5 or later version.
Lib\net461\EASendMail.dll Built with .NET Framework 4.6.1
It requires .NET Framework 4.6.1 or later version.
Lib\netcoreapp3.1\EASendMail.dll Built with .NET Core 3.1
It requires .NET Core 3.1 or later version.
Lib\net5.0\EASendMail.dll Built with .NET 5.0
It requires .NET 5.0 or later version.
Lib\net6.0\EASendMail.dll Built with .NET 6.0
It requires .NET 6.0 or later version.
Lib\netstandard2.0\EASendMail.dll Built with .NET Standard 2.0
It requires .NET Standard 2.0 or later version.
Lib\net20-cf\EASendMail.dll Built with .NET Compact Framework 2.0
It requires .NET Compact Framework 2.0, 3.5 or later version.
Lib\net35-cf\EASendMail.dll Built with .NET Compact Framework 3.5
It requires .NET Compact Framework 3.5 or later version.

The Gmail IMAP and SMTP servers have been extended to support authorization via the industry-standard OAuth 2.0 protocol. Using OAUTH protocol, user can do authentication by Google Web Login instead of inputting user and password directly in application.

Create project in Google Developers Console

To send email using Gmail OAuth in your application, you should create a project in Google Developers Console like this:

Create credentials (OAuth client id)

  • Click APIs & Services -> Dashboard -> Credentials

    google oauth Credentials
  • Click Credentials -> Create Credentials -> OAuth client ID -> Web application or Other (Desktop Application). It depends on your application type.

    google oauth Credentials
  • Input a name for your application, input your web applicaton url to receive authorization code at Authorized Redirect URIs. Desktop Application doesn’t require this step.

  • Click "Create", you will get client id and client secret

    google oauth client secret

Enable Gmail API

  • Enable Gmail API in "Library" -> Search "Gmail", then click "Gmail API" and enable it.

    enable Gmail API

Edit scopes

  • Set detail information for your project at "OAuth consent screen" -> "Edit App".

    edit Gmail oauth consent
  • Finally add "https://mail.google.com/" and "../auth/gmail.send" scopes at "OAuth consent screen" -> "Edit App" -> "Scopes for Google API".

    enable Gmail scope

API scopes

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.

  • If you use Gmail RESTFul API to send email, please only use "../auth/gmail.send" scope to avoid your app throttled;
  • If you use SMTP protocol, you should use https://mail.google.com/ scope.

Authorized Redirect URIs

If you use OAuth in a web application, you should use a web page or controller to get authorization code from Google OAuth Server. So you need to add your page or web application routing path to Authorized Redirect URIs in APIs & Services -> Dashboard -> Credentials -> OAuth 2.0 Client IDs -> Your Client ID.

Authorized Redirect URIs

Enable TLS Strong Encryption Algorithms in .NET 2.0 and .NET 4.0

Because HttpWebRequest is used to get access token from web service. If you’re using .NET framework (.NET 2.0 - 3.5 and .NET 4.x), you need to enable Strong Encryption Algorithms to request access token:

Put the following content to a file named NetStrongEncrypt.reg, right-click this file -> Merge -> Yes. You can also download it from https://www.emailarchitect.net/webapp/download/NetStrongEncrypt.zip.

Windows Registry Editor Version 5.00





Use client id and client secret to request 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 Oauth Url;
  • User inputs user and password in web authentication page, and then the Oauth server returns access token back to your application;
  • Your application uses access token to access resource on the server.
  • You can find full example codes in EASendMail Installation Path\Samples_{Programming language/Developer Tool}\Oauth project.

Access token expiration and refresh token

You don’t have to open browser to request access token every time. By default, access token expiration time is 3600 seconds, you can use the access token repeatedly before it is expired. After it is expired, you can use refresh token to refresh access token directly without opening browser. You can find full sample project in EASendMail installation path to learn how to refresh token.


You should create your client id and client secret, do not use the client id from example codes in production environment, it is used for test purpose. If you got "This app isn't verified" information, please click "Advanced" -> Go to ... for test.

C# - Send email using Google OAuth + Gmail SMTP server

Here is a console application which demonstrates how to use Google OAuth to do user authentication and send email.


This sample cannot handle the event of Web Browser is closed by user manually before authentication is completed. You can refer to the better sample project which uses Web Browser Control in EASendMail installation path.

using System;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;
using EASendMail;

namespace EASendMailKbCSharp
    class Program
        static void Main(string[] args)
            Console.WriteLine("  Sign in with Google                                             ");
            Console.WriteLine("   If you got \"This app isn't verified\" information in Web Browser, ");
            Console.WriteLine("   click \"Advanced\" -> Go to ... to continue test.");
            Console.WriteLine("Press any key to sign in...");

                Program p = new Program();
            catch (Exception ep)


        void SendMailWithXOAUTH2(string userEmail, string accessToken)
            // Gmail SMTP server address
            SmtpServer oServer = new SmtpServer("smtp.gmail.com");
            // enable SSL connection
            oServer.ConnectType = SmtpConnectType.ConnectSSLAuto;
            // Using 587 port, you can also use 465 port
            oServer.Port = 587;

            // use Gmail SMTP 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;

            // Please change recipient address to yours for test
            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!");

        // client configuration
        // You should create your client id and client secret,
        // do not use the following client id in production environment, it is used for test purpose only.
        const string clientID = "1072602369179-aru4rj97ateiho9rt4pf5i8l1r01mc16.apps.googleusercontent.com";
        const string clientSecret = "Lnw8r5FvfKFNS_CSEucbdIE-";
        const string scope = "openid%20profile%20email%20https://mail.google.com";
        const string authUri = "https://accounts.google.com/o/oauth2/v2/auth";
        const string tokenUri = "https://www.googleapis.com/oauth2/v4/token";

        static int GetRandomUnusedPort()
            var listener = new TcpListener(IPAddress.Loopback, 0);
            var port = ((IPEndPoint)listener.LocalEndpoint).Port;
            return port;

        async void DoOauthAndSendEmail()
            // Creates a redirect URI using an available port on the loopback address.
            string redirectUri = string.Format("{0}/", GetRandomUnusedPort());
            Console.WriteLine("redirect URI: " + redirectUri);

            // Creates an HttpListener to listen for requests on that redirect URI.
            var http = new HttpListener();
            Console.WriteLine("Listening ...");

            // Creates the OAuth 2.0 authorization request.
            string authorizationRequest = string.Format("{0}?response_type=code&scope={1}&redirect_uri={2}&client_id={3}",

            // Opens request in the browser.

            // Waits for the OAuth authorization response.
            var context = await http.GetContextAsync();

            // Brings the Console to Focus.

            // Sends an HTTP response to the browser.
            var response = context.Response;
            string responseString = string.Format("<html><head></head><body>Please return to the app and close current window.</body></html>");
            var buffer = Encoding.UTF8.GetBytes(responseString);
            response.ContentLength64 = buffer.Length;
            var responseOutput = response.OutputStream;
            Task responseTask = responseOutput.WriteAsync(buffer, 0, buffer.Length).ContinueWith((task) =>
                Console.WriteLine("HTTP server stopped.");

            // Checks for errors.
            if (context.Request.QueryString.Get("error") != null)
                Console.WriteLine(string.Format("OAuth authorization error: {0}.", context.Request.QueryString.Get("error")));

            if (context.Request.QueryString.Get("code") == null)
                Console.WriteLine("Malformed authorization response. " + context.Request.QueryString);

            // extracts the code
            var code = context.Request.QueryString.Get("code");
            Console.WriteLine("Authorization code: " + code);

            string responseText = await RequestAccessToken(code, redirectUri);

            OAuthResponseParser parser = new OAuthResponseParser();

            var user = parser.EmailInIdToken;
            var accessToken = parser.AccessToken;

            Console.WriteLine("User: {0}", user);
            Console.WriteLine("AccessToken: {0}", accessToken);

            SendMailWithXOAUTH2(user, accessToken);

        async Task<string> RequestAccessToken(string code, string redirectUri)
            Console.WriteLine("Exchanging code for tokens...");

            // builds the  request
            string tokenRequestBody = string.Format("code={0}&redirect_uri={1}&client_id={2}&client_secret={3}&grant_type=authorization_code",

            // sends the request
            HttpWebRequest tokenRequest = (HttpWebRequest)WebRequest.Create(tokenUri);
            tokenRequest.Method = "POST";
            tokenRequest.ContentType = "application/x-www-form-urlencoded";
            tokenRequest.Accept = "Accept=text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";

            byte[] _byteVersion = Encoding.ASCII.GetBytes(tokenRequestBody);
            tokenRequest.ContentLength = _byteVersion.Length;

            Stream stream = tokenRequest.GetRequestStream();
            await stream.WriteAsync(_byteVersion, 0, _byteVersion.Length);

                // gets the response
                WebResponse tokenResponse = await tokenRequest.GetResponseAsync();
                using (StreamReader reader = new StreamReader(tokenResponse.GetResponseStream()))
                    // reads response body
                    return await reader.ReadToEndAsync();

            catch (WebException ex)
                if (ex.Status == WebExceptionStatus.ProtocolError)
                    var response = ex.Response as HttpWebResponse;
                    if (response != null)
                        Console.WriteLine("HTTP: " + response.StatusCode);
                        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
                            // reads response body
                            string responseText = await reader.ReadToEndAsync();

                throw ex;

        // Hack to bring the Console window to front.

        [DllImport("kernel32.dll", ExactSpelling = true)]
        public static extern IntPtr GetConsoleWindow();

        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool SetForegroundWindow(IntPtr hWnd);

        public void BringConsoleToFront()

C# - Send email using Google OAuth + Gmail SMTP server in ASP.NET/ASP MVC

If you use Google OAuth in ASP.NET/ASP MVC application, you should use a ASP.NET page or ASP MVC Controller to get authorization code instead of HttpListener. You need to add your ASP.NET page or ASP MVC Controller routing path to Authorized Redirect URIs in your Google project.

// Please add http://localhost:54098/oauth/token to Authorized redirect URIs in your Google/MS Azure project.
public ActionResult Token(string code)
    // code parameter is the authorization code returned by Google OAuth server,
    // then you can use it to request AccessToken
    // just like RequestAccessToken method in previous example

You can find a full sample project in EASendMail installation path\Samples_ASPNetMvc.

TLS 1.2 protocol

TLS is the successor of SSL, more and more SMTP servers require TLS 1.2 encryption now.

If your operating system is Windows XP/Vista/Windows 7/Windows 2003/2008/2008 R2/2012/2012 R2, you need to enable TLS 1.2 protocol in your operating system like this:

Enable TLS 1.2 on Windows XP/Vista/7/10/Windows 2008/2008 R2/2012



If you have any comments or questions about above example codes, please click here to add your comments.