Visual C++ - Send email using Gmail/G Suite OAuth 2.0 in background service (service account)

By default, you need to enable ” Allowing less secure apps” in Gmail/G Suite, 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.


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++

Google Service Account

Normal OAuth requires user input user/password in Web Browser. Obviously, it is not suitable for background service. In this case, you should use google service account to access G Suite email service without user interaction. Service account only works for G Suite user, it doesn’t work for personal Gmail account.

Create project in Google Developers Console

To use “G Suite Service Account OAuth” in your application, you should create a project in Google Developers Console at first.


You can use any google user to create service account, it doesn’t require service account owner is a user in G Suite. But G Suite administrator must authorize service account in G Suite Admin Console to access user mailbox.

Create service account in current project

  • Click "Credentials" -> "Manage service accounts"

    manage service account in google developers console

    create service account in google developers console
  • Input a name for your service account, click "CREATE"

    create service account in google developers console step 1
  • In "Service account permissions", select "Project" -> "Owner" as role

    create service account in google developers console step 2
  • In "Grant users access to this service account", keep everything default and click "DONE"

    create service account in google developers console step 3

After service account is created, you should enable "Domain-wide delegation" and create service key pair to access G Suite user mailbox.

Enable “Domain-wide delegation” and create service key

  • Go back to your service account, click "Edit" -> "SHOW DOMAIN-WIDE DELEGATION", check "Enable G Suite Domain-wide Delegation", input a name for product oauth consent, click "Save".

    Enable G Suite Domain-wide Delegation
  • Go back to your service account again, click "Create Key", you can select "p12" or "json" key type, both can work well, then you will get a file which contains private key, save the file to local disk.

    Now you have created service account with key pair successfully. You can use created private key in your codes to request "access token" impersonating a user in G Suite.

    create service key
  • To access user data in G Suite, you must get authorization from G Suite administrator. You should go to service accounts list, click "View Client ID" like this:

    google oauth client id
  • Then copy your “Client ID” and service account email address, forward it to G Suite administrator for authorization.

    google oauth serivce client id

Enable Gmail API

Enable Gmail API in "Library" -> Search "Gmail", then click "Gmail API" and enable it. If you use Gmail API protocol to send email, you should enable this API, if you use SMTP protocol, you don’t have to enable it.

enable Gmail API

Authorize service account by G Suite administrator

To use service account to access user mailbox in G Suite, G Suite Administrator should authorize specified service account at first.


Important Notice: You can use any google user to create service account, it doesn’t require service account owner is a user in G Suite. But G Suite administrator must authorize service account in G Suite Admin Console to access user mailbox.

  • G Suite Administrator should open, go to Admin Console, click "Security" > API Control;

    Authorize Service Account by G Suite Administrator
  • Click Add new and enter your service account client ID.

  • Enter the client ID of the service account or OAuth2 client ID of the app.

  • In OAuth Scopes, add each scope that the application can access (should be appropriately narrow). and input, email, profile in One or More API Scopes, click "Authorize".

    If you use Gmail API protocol instead of SMTP protocol, input:, email, profile.

    manage api client access by G Suite Administrator 1

After G Suite administrator authorized service account, you can use it to access any users mailbox in G Suite domain.

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

Windows Registry Editor Version 5.00





Access token lifetime

You don’t have to request access token every time. By default, access token expiration time is 3600 seconds, you can reuse the access token repeatedly before it is expired.

Visual C++ - Send email using Gmail/G Suite OAuth 2.0 with service account - example

#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;

BOOL RequestAccessToken(const TCHAR* requestData, _bstr_t &accessToken)
        IServerXMLHTTPRequestPtr httpRequest = NULL;
        if (httpRequest == NULL)
            printf("Failed to create XML HTTP Object, please make sure you install MSXML 3.0 on your machine.\r\n");
            return FALSE;

        _bstr_t fullRequest = _bstr_t("grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=") + requestData;

        const char* postData = (const char*)fullRequest;
        LONG cdata = strlen(postData);
        LPSAFEARRAY psaHunk = ::SafeArrayCreateVectorEx(VT_UI1, 0, cdata, NULL);
        for (LONG k = 0; k < (int)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);
        _bstr_t uri("");

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

        while (httpRequest->readyState != 4) {

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

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

        IOAuthResponseParserPtr oauthParser = NULL;

        accessToken = oauthParser->AccessToken;
        if (accessToken.length() == 0)
            printf("Failed to parse access token from server response: %d %s\r\n", status, (const char*)responseText);
            return FALSE;

        return TRUE;
    catch (_com_error &ep)
        printf("Failed to get access token: %s", (const char*)ep.Description());
        return FALSE;

BOOL GenerateRequestData(const char* gsuiteUser, _bstr_t &requestData)
    // service account email address
    // You should create your client id and client secret,
    const char* serviceAccount = "";
    const char* scope = "";
    const char* aud = "";

    ISimpleJsonParserPtr jwtPtr = NULL;

    _bstr_t header = jwtPtr->JwtBase64UrlEncode(_bstr_t("{\"alg\":\"RS256\",\"typ\":\"JWT\"}"));

    // token request timestamp
    long iat = jwtPtr->GetCurrentIAT();
    // token expiration time
    long exp = iat + 3600;

    char iatBuf[MAX_PATH + 1];
    char expBuf[MAX_PATH + 1];

    sprintf_s(iatBuf, MAX_PATH, "%d", iat);
    sprintf_s(expBuf, MAX_PATH, "%d", exp);

    _bstr_t playload = _bstr_t("{");
    playload += _bstr_t("\"iss\":\"") + _bstr_t(serviceAccount) + _bstr_t("\",");
    playload += _bstr_t("\"scope\":\"") + _bstr_t(scope) + _bstr_t("\",");
    playload += _bstr_t("\"aud\":\"") + _bstr_t(aud) + _bstr_t("\",");
    playload += _bstr_t("\"exp\":") + _bstr_t(expBuf) + _bstr_t(",");
    playload += _bstr_t("\"iat\":") + _bstr_t(iatBuf) + _bstr_t(",");
    playload += _bstr_t("\"sub\":\"") + _bstr_t(gsuiteUser) + _bstr_t("\"");
    playload += _bstr_t("}");

    playload = jwtPtr->JwtBase64UrlEncode(playload);

    ICertificatePtr cert = NULL;

    // In web application, use CRYPT_MACHINE_KEYSET
    if (cert->LoadPFXFromFile(_bstr_t("D:\\MyData\\oauth-77dac4d192ec.p12"),
        _bstr_t("notasecret"), CRYPT_USER_KEYSET) == VARIANT_FALSE)
        printf("Failed to load service account certificate!");
        return FALSE;

    _bstr_t signature = jwtPtr->SignRs256(cert, header + _bstr_t(".") + playload);
    if (signature.length() == 0)
        printf("Failed to sign request data!");
        return FALSE;

    requestData = header + _bstr_t(".") + playload + _bstr_t(".") + signature;
    return TRUE;

void SendEmail()

    // gsuiteUser is the full email address of the user in GSuite, for example: user@gsuitedomain
    const char* gsuiteUser = "";

    _bstr_t requestData, accessToken;
    if (!GenerateRequestData(gsuiteUser, requestData))

    // request access token from Google server by service account
    // withou user interaction
    if (!RequestAccessToken((const TCHAR*)requestData, accessToken))

    IMailPtr oSmtp = NULL;
    oSmtp->LicenseCode = _bstr_t("TryIt");

    // Gmail SMTP server address
    oSmtp->ServerAddr = _bstr_t("");
    oSmtp->ServerPort = 587;

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

    // Gmail OAUTH type
    oSmtp->AuthType = AuthXoauth2;
    oSmtp->UserName = _bstr_t(gsuiteUser);
    oSmtp->Password = accessToken;

    oSmtp->FromAddr = _bstr_t(gsuiteUser);

    // Please change recipient address to yours for test
    oSmtp->AddRecipient(_bstr_t("Support Team"),
        _bstr_t(""), 0);

    oSmtp->Subject = _bstr_t("Test email using G suite service account");
    oSmtp->BodyText = _bstr_t("Hello, this is a test....");

    if (oSmtp->SendMail() == 0)
        printf("Message delivered!");
        printf((const char*)oSmtp->GetLastErrDescription());

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



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