Visual C++ - 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.

Installation

EASendMail is a SMTP component which supports all operations of SMTP/ESMTP protocols (RFC 821, RFC 822, RFC 2554). Before you can use the following example codes, you should download the EASendMail Installer and install it on your machine at first.

Add Reference

To use EASendMail SMTP ActiveX Object in your C++ project, the first step is “Add header files of EASendMail to your project”. Please go to C:\Program Files\EASendMail\Include\tlh or C:\Program Files (x86)\EASendMail\Include\tlh folder, find easendmailobj.tlh and easendmailobj.tli, and then copy these files to your project folder.

add reference in Visual C++

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

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 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 full example codes in EASendMail Installation Path\Samples_{Programming language/Developer Tool}\Oauth project.

Important

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.

Visual 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.

Note

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.

#include "stdafx.h" // pre-compile header

#include "C:\Program Files (x86)\EASendMail\Include\tlh\EASendMailObj.tlh"
using namespace EASendMailObjLib;

#include "C:\Program Files (x86)\EASendMail\Include\tlh\msxml3.tlh"
using namespace MSXML2;


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 SendMailWithXOAUTH2(const char* lpszEmail, const char* lpszAccessToken)
{
    IMailPtr oSmtp = NULL;
    oSmtp.CreateInstance(__uuidof(EASendMailObjLib::Mail));
    oSmtp->LicenseCode = _bstr_t("TryIt");

    // Gmail SMTP server address
    oSmtp->ServerAddr = _bstr_t("smtp.gmail.com");
    // Using 587 port, you can also use 465 or 25 port
    oSmtp->ServerPort = 587;

    // Enable SSL/TLS connection
    oSmtp->ConnectType = ConnectSSLAuto;

    //  XOAUTH 2.0 authentication
    oSmtp->AuthType = AuthXoauth2;
    // set user authentication
    oSmtp->UserName = lpszEmail;
    // use access token as password
    oSmtp->Password = lpszAccessToken;

    // Your email address
    oSmtp->FromAddr = lpszEmail;
    // Please change recipient address to yours for test
    oSmtp->AddRecipient(_bstr_t("Support Team"),
        _bstr_t("support@emailarchitect.net"), 0);

    oSmtp->Subject = _bstr_t("test email from gmail account using OAUTH 2");
    oSmtp->BodyText = _bstr_t("Hello, this is a test email ....");

    printf("start to send email using OAUTH 2.0 ...\n");

    if (oSmtp->SendMail() == 0)
        printf("The email has been submitted to server successfully!\n");
    else
        printf("%s\n", (const char*)oSmtp->GetLastErrDescription());
}


// 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 char* clientID = "1072602369179-aru4rj97ateiho9rt4pf5i8l1r01mc16.apps.googleusercontent.com";
const char* clientSecret = "Lnw8r5FvfKFNS_CSEucbdIE-";
const char* scope = "openid%20profile%20email%20https://mail.google.com";
const char* authUri = "https://accounts.google.com/o/oauth2/v2/auth";
const char* tokenUri = "https://www.googleapis.com/oauth2/v4/token";

// path?parameter1=value1&parameter2=value2#anchor;
_bstr_t ParseParameter(const char* uri, const char* key)
{
    _bstr_t value = "";
    if (uri == NULL || key == NULL)
        return value;

    const char* p = strchr(uri, '?');
    p = (p != NULL) ? p + 1 : uri;

    do
    {
        if (_strnicmp(p, key, strlen(key)) == 0)
        {
            p += strlen(key);
            const char* pend = strpbrk(p, "&#");
            if (pend != NULL)
            {
                char* buf = new char[pend - p + 1];
                memcpy(buf, p, pend - p);
                buf[pend - p] = '\0';
                value = buf;

                delete[] buf;
            }
            else
            {
                value = p;
            }

            return value;
        }

        p = strchr(p, '&');
        if (p) p++;

    } while (p);

    return value;
}

_bstr_t RequestAccessToken(const char* code, const char* redirectUri)
{
    printf("Exchanging code for tokens...\n");

    _bstr_t responseText = "";
    try
    {
        IServerXMLHTTPRequestPtr httpRequest = NULL;
        httpRequest.CreateInstance(__uuidof(MSXML2::ServerXMLHTTP));
        if (httpRequest == NULL)
        {
            printf("Failed to create XML HTTP Object, please make sure you install MSXML 3.0 on your machine.\n");
            return responseText;
        }

        _bstr_t tokenRequestBody = "code=";
        tokenRequestBody += code;
        tokenRequestBody += "&redirect_uri=";
        tokenRequestBody += redirectUri;
        tokenRequestBody += "&client_id=";
        tokenRequestBody += clientID;
        tokenRequestBody += "&client_secret=";
        tokenRequestBody += clientSecret;
        tokenRequestBody += "&grant_type=authorization_code";

        const char* postData = (const char*)tokenRequestBody;
        long cdata = (long)strlen(postData);
        LPSAFEARRAY psaHunk = ::SafeArrayCreateVectorEx(VT_UI1, 0, cdata, NULL);
        for (long k = 0; k < cdata; k++)
        {
            BYTE ch = (BYTE)postData[k];
            ::SafeArrayPutElement(psaHunk, &k, &ch);
        }

        _variant_t requestBuffer;
        requestBuffer.vt = (VT_ARRAY | VT_UI1);
        requestBuffer.parray = psaHunk;

        _variant_t async(true);

        httpRequest->setOption((MSXML2::SERVERXMLHTTP_OPTION)2, 13056);
        httpRequest->open(L"POST", _bstr_t(tokenUri), async, vtMissing, vtMissing);
        httpRequest->setRequestHeader(L"Content-Type", L"application/x-www-form-urlencoded");
        httpRequest->send(requestBuffer);

        while (httpRequest->readyState != 4) {
            httpRequest->waitForResponse(1);
        }

        long status = httpRequest->status;
        responseText = httpRequest->responseText;

        if (status < 200 || status >= 300)
        {
            printf("Failed to get access token from server: %d %s\n", status, (const char*)responseText);
        }

        return responseText;
    }
    catch (_com_error &ep)
    {
        printf("Failed to get access token: %s\n", (const char*)ep.Description());
    }

    return responseText;
}

void DoOauthAndSendEmail()
{
    IHttpListenerPtr httpListener = NULL;
    httpListener.CreateInstance(__uuidof(EASendMailObjLib::HttpListener));

    // Creates a redirect URI using an available port on the loopback address.
    if (httpListener->Create("127.0.0.1", 0) == VARIANT_FALSE)
    {
        printf("Failed to listen on %s\n", (const char*)httpListener->GetLastError());
        return;
    }

    char szUri[MAX_PATH + 1];
    sprintf_s(szUri, MAX_PATH, "http://127.0.0.1:%d", httpListener->ListenPort);
    printf("listen on %s ...\n", szUri);

    // Creates the OAuth 2.0 authorization request.
    _bstr_t authorizationRequest = authUri;
    authorizationRequest += "?response_type=code&scope=";
    authorizationRequest += scope;
    authorizationRequest += "&redirect_uri=";
    authorizationRequest += szUri;
    authorizationRequest += "&client_id=";
    authorizationRequest += clientID;

    printf("open %s ...\n", (const char*)authorizationRequest);

    // Opens request in the browser.
    IBrowserUiPtr browserUi = NULL;
    browserUi.CreateInstance(__uuidof(EASendMailObjLib::BrowserUi));
    browserUi->OpenUrl(authorizationRequest);

    // Waits for the OAuth authorization response.
    if (httpListener->GetRequestUrl(-1) == VARIANT_FALSE)
    {
        printf("Failed to get authorization response %s\n", (const char*)httpListener->GetLastError());
        return;
    }

    // Brings the Console to Focus.
    SetForegroundWindow(GetConsoleWindow());

    // Send response and stop http listener.
    httpListener->SendResponse(_bstr_t("200"),
        _bstr_t("text/html; charset=utf-8"),
        "<html><head></head><body>Please return to the app and close current window.</body></html>");
    httpListener->Close();

    _bstr_t requestUri = httpListener->RequestUrl;
    printf("RequestUri: %s\n", (const char*)requestUri);

    // Checks for errors.
    _bstr_t error = ParseParameter((const char*)requestUri, "error=");
    if (error.length())
    {
        printf("OAuth authorization error: %s\n", (const char*)error);
        return;
    }

    // Check authorization code
    _bstr_t code = ParseParameter((const char*)requestUri, "code=");
    if (code.length() == 0)
    {
        printf("Malformed authorization response: %s\n", (const char*)requestUri);
        return;
    }

    printf("\nAuthorization code: %s\n", (const char*)code);

    _bstr_t responseText = RequestAccessToken((const char*)code, szUri);
    printf("%s\n", (const char*)responseText);

    IOAuthResponseParserPtr parser = NULL;
    parser.CreateInstance(__uuidof(EASendMailObjLib::OAuthResponseParser));
    parser->Load(responseText);

    _bstr_t user = parser->EmailInIdToken;
    _bstr_t accessToken = parser->AccessToken;

    if (accessToken.length() == 0)
    {
        printf("Failed to request access token, return!\n");
        return;
    }

    printf("User: %s\n", (const char*)user);
    printf("AccessToken: %s\n", (const char*)accessToken);

    SendMailWithXOAUTH2((const char*)user, (const char*)accessToken);
}

int main()
{
    ::CoInitialize(NULL);

    printf(
        "+------------------------------------------------------------------+\n"
        "  Sign in with Google                                             \n"
        "   If you got \"This app isn't verified\" information in Web Browser, \n"
        "   click \"Advanced\" -> Go to ... to continue test.\n"
        "+------------------------------------------------------------------+\n\n");

    printf("Press ENTER key to sign in...\n");
    getchar();

    DoOauthAndSendEmail();

    printf("Press ENTER key to quit...\n");
    getchar();

    return 0;
}

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

Appendix

Comments

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