Visual C++ - Retrieve Email using Gmail/G Suite OAuth 2.0 + IMAP4 Protocol in Background Service (Service Account)

By default, you need to enable ” Allowing less secure apps” in Gmail/G Suite, then you can retrieve email with user/password IMAP4 authentication.

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

Installation

Before you can use the following sample codes, you should download the EAGetMail Installer and install it on your machine at first. Full sample projects are included in this installer.

Add Reference

To better demonstrate how to retrieve email and parse email, let’s create a Visual C++ console project named “receiveemail” at first, and then add the reference of EAGetMail in your project.

Visual C++ console project

To use EAGetMail POP3 & IMAP4 ActiveX Object in your project, the first step is “Add header files of EAGetMail to your project”. Please go to C:\Program Files\EAGetMail\Include\tlh or C:\Program Files (x86)\EAGetMail\Include\tlh folder, find eagetmailobj.tlh and eagetmailobj.tli, and then copy these files to your project folder. You can start to use it to retrieve email and parse email in your project.

add reference in 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.

Important

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
  • Click "CREATE SERVICE ACCOUNT"

    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

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 admin.google.com, go to Admin Console, click "Security";

    Authorize Service Account by G Suite Administrator
  • Click "Advanced settings" -> "Manage API client access";

    manage api client access by G Suite Administrator
  • Input service account "Client ID" in Client Name, and input "https://mail.google.com/,email,profile" in One or More API Scopes, click "Authorize".

    If you use Gmail API protocol instead of SMTP protocol, input: "https://www.googleapis.com/auth/gmail.send,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.

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.

Visual C++ - Retrieve Email using Gmail/G Suite OAuth 2.0 from IMAP4 Server with Service Account - Example

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

#include <stdio.h>
#include <tchar.h>

#include "C:\Program Files (x86)\EAGetMail\Include\tlh\EAGetMailObj.tlh"
using namespace EAGetMailObjLib;

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


const int MailServerPop3 = 0;
const int MailServerImap4 = 1;
const int MailServerEWS = 2;
const int MailServerDAV = 3;

const int MailServerAuthLogin = 0;
const int MailServerAuthCRAM5 = 1;
const int MailServerAuthNTLM = 2;
const int MailServerAuthXOAUTH2 = 3;

const int GetMailInfos_All = 1;
const int GetMailInfos_NewOnly = 2;
const int GetMailInfos_ReadOnly = 4;
const int GetMailInfos_SeqRange = 8;
const int GetMailInfos_UIDRange = 16;
const int GetMailInfos_PR_ENTRYID = 32;
const int GetMailInfos_DateRange = 64;
const int GetMailInfos_OrderByDateTime = 128;

DWORD  _getCurrentPath(LPTSTR lpPath, DWORD nSize)
{
    DWORD dwSize = ::GetModuleFileName(NULL, lpPath, nSize);
    if (dwSize == 0 || dwSize == nSize)
    {
        return 0;
    }

    // Change file name to current full path
    LPCTSTR psz = _tcsrchr(lpPath, _T('\\'));
    if (psz != NULL)
    {
        lpPath[psz - lpPath] = _T('\0');
        return _tcslen(lpPath);
    }

    return 0;
}

BOOL RequestAccessToken(const TCHAR* requestData, _bstr_t &accessToken)
{
    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.\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("https://oauth2.googleapis.com/token");

        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");
        httpRequest->send(requestBuffer);

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

        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;
        oauthParser.CreateInstance(__uuidof(EAGetMailObjLib::OAuthResponseParser));
        oauthParser->Load(responseText);

        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 = "xxxx@xxxx.iam.gserviceaccount.com";
    const char* scope = "https://mail.google.com/";
    const char* aud = "https://oauth2.googleapis.com/token";

    ISimpleJsonParserPtr jwtPtr = NULL;
    jwtPtr.CreateInstance(__uuidof(EAGetMailObjLib::SimpleJsonParser));

    _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;
    cert.CreateInstance(__uuidof(EAGetMailObjLib::Certificate));

    // In web application, use CRYPT_MACHINE_KEYSET
    if (cert->LoadFromFile(_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 RetrieveEmail()
{
    ::CoInitialize(NULL);

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

    _bstr_t requestData, accessToken;
    if (!GenerateRequestData(gsuiteUser, requestData))
    {
        return;
    }

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

    try
    {
        TCHAR szPath[MAX_PATH + 1];
        _getCurrentPath(szPath, MAX_PATH);

        TCHAR szMailBox[MAX_PATH + 1];
        wsprintf(szMailBox, _T("%s\\inbox"), szPath);

        // Create a folder to store emails
        ::CreateDirectory(szMailBox, NULL);


        IMailServerPtr oServer = NULL;
        oServer.CreateInstance(__uuidof(EAGetMailObjLib::MailServer));

        // Gmail IMAP server address
        oServer->Server = _bstr_t("imap.gmail.com");
        oServer->User = _bstr_t(gsuiteUser);

        // Use access token as password
        oServer->Password = accessToken;
        // Use OAUTH 2.0
        oServer->AuthType = MailServerAuthXOAUTH2;
        // Use IMAP4 Protocol
        oServer->Protocol = MailServerImap4;

        // Enable SSL Connection
        oServer->SSLConnection = VARIANT_TRUE;
        // Set IMAP4 SSL Port
        oServer->Port = 993;

        IMailClientPtr oClient = NULL;
        oClient.CreateInstance(__uuidof(EAGetMailObjLib::MailClient));
        oClient->LicenseCode = _T("TryIt");

        _tprintf(_T("Connecting %s ...\r\n"), (const TCHAR*)oServer->Server);
        oClient->Connect(oServer);

        // Get new email only, if you want to get all emails, please remove this line
        oClient->GetMailInfosParam->GetMailInfosOptions = GetMailInfos_NewOnly;

        IMailInfoCollectionPtr infos = oClient->GetMailInfoList();
        _tprintf(_T("Total %d emails\r\n"), infos->Count);

        for (long i = 0; i < infos->Count; i++)
        {
            IMailInfoPtr pInfo = infos->GetItem(i);

                        _tprintf(_T("Index: %d; Size: %d; UIDL: %s\r\n\r\n"),
                                pInfo->Index, pInfo->Size, (const TCHAR*)pInfo->UIDL);

            TCHAR szFile[MAX_PATH + 1];
            // Generate a random file name by current local datetime,
            // You can use your method to generate the filename if you do not like it
            SYSTEMTIME curtm;
            ::GetLocalTime(&curtm);
            ::wsprintf(szFile, _T("%s\\%04d%02d%02d%02d%02d%02d%03d%d.eml"),
                szMailBox,
                curtm.wYear,
                curtm.wMonth,
                curtm.wDay,
                curtm.wHour,
                curtm.wMinute,
                curtm.wSecond,
                curtm.wMilliseconds,
                i);

            // Receive email from POP3 server
            IMailPtr oMail = oClient->GetMail(pInfo);

            _tprintf(_T("From: %s\r\n"), (const TCHAR*)oMail->From->Address);
            _tprintf(_T("Subject: %s\r\n"), (const TCHAR*)oMail->Subject);

            // Save email to local disk
            oMail->SaveAs(szFile, VARIANT_TRUE);

            // Mark email as read to prevent retrieving this email again.
            oClient->MarkAsRead(pInfo, VARIANT_TRUE);

            // If you want to delete current email, please use Delete method instead of MarkAsRead
            // oClient->Delete(pInfo);
        }

        // Delete method just mark the email as deleted,
        // Quit method expunge the emails from server exactly.
        oClient->Quit();
    }
    catch (_com_error &ep)
    {
        _tprintf(_T("Error: %s"), (const TCHAR*)ep.Description());
    }
}

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

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