Using Gmail/GSuite IMAP OAUTH with Service Account


The Gmail/GSuite 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 Gmail Web OAuth instead of inputting user and password directly in application.

You can click here to learn more detail about "OAUTH/XOAUTH2 with Gmail SMTP Server".


Google Service Account

Normal OAUTH requires user input user/password for authentication. 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 your 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 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.


Create service account under current project

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


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.

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


Use service account to send email impersonating user in G Suite Domain

The following examples demonstrate how to send email with service account + G Suite OAUTH

Example

[VB6 - Use service account to retrieve email impersonating user in G Suite Domain]

Const MailServerPop3 = 0
Const MailServerImap4 = 1
Const MailServerEWS = 2
Const MailServerDAV = 3

Const MailServerAuthLogin = 0
Const MailServerAuthCRAM5 = 1
Const MailServerAuthNTLM = 2
Const MailServerAuthXOAUTH2 = 3

Const CRYPT_MACHINE_KEYSET = 32 
Const CRYPT_USER_KEYSET = 4096 

Private Function GenerateRequestData(GsuiteUser) 

    GenerateRequestData = ""

    ' service account email address
    Const serviceAccount = "xxxxxx@xxxxxx.iam.gserviceaccount.com"
    Const scope = "https://mail.google.com/"
    Const aud = "https://oauth2.googleapis.com/token"

    Dim jwt As New EAGetMailObjLib.SimpleJsonParser

    Dim header, playload
    header = jwt.JwtBase64UrlEncode("{""alg"":""RS256"",""typ"":""JWT""}")

    Dim iat, exp
    ' token request timestamp
    iat = jwt.GetCurrentIAT()
    ' token expiration time
    exp = iat + 3600

    playload = "{"
    playload = playload & """iss"":""" & serviceAccount & ""","
    playload = playload & """scope"":""" & scope & ""","
    playload = playload & """aud"":""" & aud & ""","
    playload = playload & """exp"":" & exp & ","
    playload = playload & """iat"":" & iat & ","
    playload = playload & """sub"":""" & gsuiteUser & """"
    playload = playload & "}"

    playload = jwt.JwtBase64UrlEncode(playload)

    Dim cert As New EAGetMailObjLib.Certificate

    ' In web application, use CRYPT_MACHINE_KEYSET
    cert.LoadFromFile "D:\myfolder\myoauth-77dec4d192ec.p12", "notasecret", CRYPT_USER_KEYSET

    Dim signature
    signature = jwt.SignRs256(cert, header & "." & playload)
    If signature = "" Then
        MsgBox "Failed to sign request data!"
        Exit Function
    End If
    
    Dim dataToPost

    dataToPost = header & "." & playload & "." & signature
    GenerateRequestData = dataToPost

End Function

Private Function RequestAccessToken(requestData)
    RequestAccessToken = ""

    If requestData = "" Then
        Exit Function
    End If

On Error GoTo ErrorHandle

    Dim httpRequest
    Set httpRequest = CreateObject("MSXML2.ServerXMLHTTP")
    
    requestData = "grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=" & requestData
    
    httpRequest.setOption 2, 13056
    httpRequest.Open "POST", "https://oauth2.googleapis.com/token", True
    httpRequest.setRequestHeader "Content-Type", "application/x-www-form-urlencoded"
    httpRequest.Send requestData
    
    Do While httpRequest.ReadyState <> 4
        DoEvents
        httpRequest.waitForResponse (1)
    Loop
    
    Dim Status
    Status = httpRequest.Status
    
    If Status < 200 Or Status >= 300 Then
        MsgBox "Failed to get access token from server."
        MsgBox httpRequest.responseText
        Exit Function
    End If

    Dim result
    result = httpRequest.responseText
    
    Dim oauthParser As New EAGetMailObjLib.OAuthResponseParser
    oauthParser.Load result
    
    Dim accessToken
    accessToken = oauthParser.AccessToken
    
    If accessToken = "" Then
        MsgBox "Failed to parse access token from server response."
        Exit Function
    End If

    RequestAccessToken = accessToken
    Exit Function

ErrorHandle:
    MsgBox "Failed to request access token." & Err.Description
        
End Function

Public Sub ReceiveMail()
    
On Error GoTo ErrorHandle
    ' GsuiteUser is the full email address of the user in GSuite, user@gsuitedomain
    Dim GsuiteUser
    GsuiteUser = "user@gsuitedomain"

    Dim accessToken As String
    ' request access token from Google server by service account
    ' withou user interaction
    accessToken = RequestAccessToken(GenerateRequestData(GsuiteUser))

    If accessToken = "" Then
        Exit Sub
    End If

    Dim oClient As New EAGetMailObjLib.MailClient
    oClient.LicenseCode = "TryIt"
    
    Dim oServer As New EAGetMailObjLib.MailServer
    oServer.Server = "imap.gmail.com"
    oServer.User = GsuiteUser
    oServer.Password = accessToken
    oServer.Protocol = MailServerImap4
    oServer.AuthType = MailServerAuthXOAUTH2
    
    oServer.SSLConnection = True
    oServer.Port = 993 'SSL IMAP4

    oClient.Connect oServer

    Dim infos
    Set infos = oClient.GetMailInfoList()
    
    Dim i
    For i = 0 To infos.Count - 1
        Dim info
        Set info = infos.Item(i)
        
        MsgBox "UIDL: " & info.UIDL
        MsgBox "Index: " & info.Index
        MsgBox "Size: " & info.Size

        MsgBox "Read: " & info.Read
        MsgBox "Deleted: " & info.Deleted
                    
        Dim oMail
        Set oMail = oClient.GetMail(info)
        'Save mail to local
        oMail.SaveAs "d:\tempfolder\" & i & ".eml", True

        ' Delete email from server
        oClient.Delete info
    Next

    oClient.Quit
    Exit Sub

ErrorHandle:
    ''Error handle
    MsgBox Err.Description
    
    oClient.Close
End Sub


[Visual C++ - Use service account to retrieve email impersonating user in G Suite Domain] #include "stdafx.h" #include <tchar.h> #include <Windows.h> #include "eagetmailobj.tlh" using namespace EAGetMailObjLib; #include "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; BOOL RequestAccessToken(const TCHAR* requestData, _bstr_t &accessToken) { try { IServerXMLHTTPRequestPtr httpRequest = NULL; httpRequest.CreateInstance(__uuidof(MSXML2::ServerXMLHTTP)); if (httpRequest == NULL) { _tprintf(_T("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(_T("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) { _tprintf(_T("Failed to get access token from server: %d %s\r\n"), status, (const TCHAR*)responseText); return FALSE; } IOAuthResponseParserPtr oauthParser = NULL; oauthParser.CreateInstance(__uuidof(EAGetMailObjLib::OAuthResponseParser)); oauthParser->Load(responseText); accessToken = oauthParser->AccessToken; if (accessToken.length() == 0) { _tprintf(_T("Failed to parse access token from server response: %d %s\r\n"), status, (const TCHAR*)responseText); return FALSE; } return TRUE; } catch (_com_error &ep) { _tprintf(_T("Failed to get access token: %s"), (const TCHAR*)ep.Description()); return FALSE; } } BOOL GenerateRequestData(const TCHAR* gsuiteUser, _bstr_t &requestData) { // service account email address const TCHAR* serviceAccount = _T("xxxx@xxxx.iam.gserviceaccount.com"); const TCHAR* scope = _T("https://mail.google.com/"); const TCHAR* aud = _T("https://oauth2.googleapis.com/token"); ISimpleJsonParserPtr jwtPtr = NULL; jwtPtr.CreateInstance(__uuidof(EAGetMailObjLib::SimpleJsonParser)); _bstr_t header = jwtPtr->JwtBase64UrlEncode(_T("{\"alg\":\"RS256\",\"typ\":\"JWT\"}")); // token request timestamp long iat = jwtPtr->GetCurrentIAT(); // token expiration time long exp = iat + 3600; TCHAR iatBuf[MAX_PATH + 1]; TCHAR expBuf[MAX_PATH + 1]; _stprintf_s(iatBuf, MAX_PATH, _T("%d"), iat); _stprintf_s(expBuf, MAX_PATH, _T("%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 cert->LoadFromFile(_T("D:\\MyData\\oauth-77dac4d192ec.p12"), _T("notasecret"), CRYPT_USER_KEYSET)); _bstr_t signature = jwtPtr->SignRs256(cert, header + _bstr_t(".") + playload); if (signature.length() == 0) { _tprintf(_T("Failed to sign request data!")); return FALSE; } requestData = header + _bstr_t(".") + playload + _bstr_t(".") + signature; return TRUE; } void ReceiveMail() { ::CoInitialize(NULL); try { // gsuiteUser is the full email address of the user in GSuite, for example: user@gsuitedomain _bstr_t gsuiteUser = _T("user@gsuitedomain"); _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; } IMailClientPtr oClient; oClient.CreateInstance(__uuidof(EAGetMailObjLib::MailClient)); IMailServerPtr oServer; oServer.CreateInstance(__uuidof(EAGetMailObjLib::MailServer)); oClient->LicenseCode = _T("TryIt"); oServer->Server = _T("imap.gmail.com"); oServer->User = gsuiteUser; oServer->Password = accessToken; oServer->Protocol = MailServerImap4; oServer->AuthType = MailServerAuthXOAUTH2; oServer->SSLConnection = VARIANT_TRUE; oServer->Port = 993; oClient->Connect(oServer); IMailInfoCollectionPtr infos = oClient->GetMailInfoList(); for(long i = 0; i < infos->Count; i++) { IMailInfoPtr pInfo = infos->GetItem(i); _tprintf(_T("UIDL: %s\r\n"), (TCHAR*)pInfo->UIDL); _tprintf(_T("Index: %d\r\n"), pInfo->Index); _tprintf(_T("Size: %d\r\n"), pInfo->Size); _tprintf(_T("Read: %s\r\n"), (pInfo->Read == VARIANT_TRUE)?_T("TRUE"):_T("FALSE")); _tprintf(_T("Deleted: %s\r\n"), (pInfo->Deleted == VARIANT_TRUE)?_T("TRUE"):_T("FALSE")); IMailPtr oMail = oClient->GetMail(pInfo); TCHAR szFile[MAX_PATH+1]; memset(szFile, 0, sizeof(szFile)); ::wsprintf(szFile, _T("d:\\tempfolder\\%d.eml"), i); //save to local disk oMail->SaveAs(szFile, VARIANT_TRUE); oMail.Release(); // delete email from server oClient->Delete(pInfo); } oClient->Quit(); } catch(_com_error &ep) { _tprintf(_T("ERROR: %s\r\n"), (TCHAR*)ep.Description()); } }
[Delphi - Use service account to retrieve email impersonating user in G Suite Domain] const MailServerPop3 = 0; MailServerImap4 = 1; MailServerEWS = 2; MailServerDAV = 3; // Auth type MailServerAuthLogin = 0; MailServerAuthCRAM5 = 1; MailServerAuthNTLM = 2; MailServerAuthXOAUTH2 = 3; CRYPT_MACHINE_KEYSET = 32; CRYPT_USER_KEYSET = 4096; function TForm1.RequestAccessToken(requestData: WideString): WideString; var httpRequest: TServerXMLHTTP; oauthParser: TOAuthResponseParser; fullRequest: OleVariant; status: integer; responseText: WideString; accessToken: WideString; begin result := ''; httpRequest := TServerXMLHTTP.Create(Application); fullRequest := 'grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion='; fullRequest := fullRequest + requestData; httpRequest.setOption(2, 13056); httpRequest.open('POST', 'https://oauth2.googleapis.com/token', true); httpRequest.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded'); httpRequest.send(fullRequest); while( httpRequest.readyState <> 4 ) do begin try httpRequest.waitForResponse(1); Application.ProcessMessages(); except ShowMessage('Server response timeout (access token).'); exit; end; end; status := httpRequest.status; responseText := httpRequest.responseText; if (status < 200) or (status >= 300) then begin ShowMessage('Failed to get access token from server.' + responseText); exit; end; oauthParser := TOAuthResponseParser.Create(Application); oauthParser.Load(responseText); accessToken := oauthParser.AccessToken; if accessToken = '' then begin ShowMessage('Failed to parse access token from server response.'); exit; end; result := accessToken; end; function TForm1.GenerateRequestData(gsuiteUser: WideString): WideString; const // service account email address serviceAccount: WideString = 'xxxx@xxxx.iam.gserviceaccount.com'; scope: WideString = 'https://mail.google.com/'; aud: WideString = 'https://oauth2.googleapis.com/token'; var jwt: TSimpleJsonParser; cert: TCertificate; pfxPath: WideString; header: WideString; playload: WideString; signature: WideString; iat, exp: integer; begin result := ''; SetThreadLocale(GetSystemDefaultLCID()); jwt := TSimpleJsonParser.Create(application); header := jwt.JwtBase64UrlEncode('{"alg":"RS256","typ":"JWT"}'); // token request timestamp iat := jwt.GetCurrentIAT(); // token expiration time exp := iat + 3600; playload := '{'; playload := playload + '"iss":"' + serviceAccount + '",'; playload := playload + '"scope":"' + scope + '",'; playload := playload + '"aud":"' + aud + '",'; playload := playload + '"exp":' + IntToStr(exp) + ','; playload := playload + '"iat":' + IntToStr(iat) + ','; playload := playload + '"sub":"' + gsuiteUser + '"'; playload := playload + '}'; playload := jwt.JwtBase64UrlEncode(playload); cert := TCertificate.Create(application); // load service account certificate to sign request data pfxPath := 'D:\MyData\GSuite\outh-77aec4d192ec.p12'; cert.LoadFromFile(pfxPath, 'notasecret', CRYPT_USER_KEYSET); signature := jwt.SignRs256(cert.DefaultInterface, header + '.' + playload); if signature = '' then begin ShowMessage('Failed to sign request data!'); exit; end; result := header + '.' + playload + '.' + signature; end; procedure TForm1.ReceiveMail(); var gsuiteUser, accessToken: WideString; oServer: TMailServer; oClient: TMailClient; oTools: TTools; oMail: IMail; infos: IMailInfoCollection; oInfo: IMailInfo; localInbox, fileName: WideString; i: Integer; begin try gsuiteUser := 'user@gsuitedomain'; // request access token with service account // gsuiteUser is the full email address of the user in GSuite, for example: user@gsuitedomain accessToken := RequestAccessToken(GenerateRequestData(gsuiteUser)); if accessToken = '' then exit; // set current thread code page to system default code page. SetThreadLocale(GetSystemDefaultLCID()); oTools := TTools.Create(Application); // Create a folder named "inbox" under // current directory to store the email files localInbox := GetCurrentDir() + '\inbox'; oTools.CreateFolder(localInbox); oServer := TMailServer.Create(Application); oServer.Server := 'imap.gmail.com'; oServer.User := gsuiteUser; oServer.Password := accessToken; oServer.Protocol := MailServerImap4; oServer.AuthType := MailServerAuthXOAUTH2; // Enable SSL Connection oServer.SSLConnection := true; oServer.Port := 993; oClient := TMailClient.Create(Application); oClient.LicenseCode := 'TryIt'; oClient.Connect1(oServer.DefaultInterface); ShowMessage('Connected!'); infos := oClient.GetMailInfoList(); ShowMessage(Format('Total %d email(s)', [infos.Count])); for i := 0 to infos.Count - 1 do begin oInfo := infos.Item[i]; ShowMessage(Format('Index: %d; Size: %d; UIDL: ' + oInfo.UIDL, [oInfo.Index, oInfo.Size])); // Generate a random file name by current local datetime, // You can use your method to generate the filename if you do not like it fileName := localInbox + '\' + oTools.GenFileName(i) + '.eml'; // Receive email from IMAP server oMail := oClient.GetMail(oInfo); ShowMessage('From: ' + oMail.From.Address + #13#10 + 'Subject: ' + oMail.Subject); // Save email to local disk oMail.SaveAs(fileName, true); // Mark email as deleted from IMAP server oClient.Delete(oInfo); end; // Quit and expunge emails marked as deleted from IMAP server oClient.Quit; except on ep:Exception do ShowMessage('Error: ' + ep.Message); end; end; end.

Remarks

You don't have to request access token every time, once you get an access token, it is valid within 3600 seconds.

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.

Remarks

You don't have to request access token every time, once you get an access token, it is valid within 3600 seconds.

If you don't want to use OAUTH 2.0, Gmail also supports traditional IMAP authentication, but you need to enable Allowing less secure apps or Sign in using App Passwords.

See Also

Using EAGetMail POP3 & IMAP4 ActiveX Object
Registration-free COM with Manifest File
User Authentication and SSL Connection
Enable TLS 1.2 on Windows XP/2003/2008/7/2008 R2
Using Gmail IMAP4 OAUTH
Using Office365 EWS OAUTH
Using Office365 EWS OAUTH in Background Service
Using Hotmail IMAP4 OAUTH
Digital Signature and E-mail Encryption/Decryption
Unique Identifier (UIDL) in POP3 and IMAP4 protocol
Parse Bounced Email (delivery-report)
Work with winmail.dat (TNEF Parser)
EAGetMail ActiveX Object References
EAGetMail POP3 & IMAP4 Component Samples

Online Tutorials

VB6 - Retrieve Email using Google/Gmail OAuth 2.0 Authentication + IMAP Protocol
VB6 -Retrieve Email using Gmail/G Suite OAuth 2.0 + IMAP4 Protocol in Background Service (Service Account)
VB6 -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + IMAP4 Protocol from Hotmail/Outlook Account
VB6 -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS/IMAP4 Protocol from Office 365 Account
VB6 -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 in Background Service

Delphi - Retrieve Email using Google/Gmail OAuth 2.0 Authentication + IMAP Protocol
Delphi -Retrieve Email using Gmail/G Suite OAuth 2.0 + IMAP4 Protocol in Background Service (Service Account)
Delphi -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + IMAP4 Protocol from Hotmail/Outlook Account
Delphi -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS/IMAP4 Protocol from Office 365 Account
Delphi -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 in Background Service

VC++ - Retrieve Email using Google/Gmail OAuth 2.0 Authentication + IMAP Protocol
VC++ -Retrieve Email using Gmail/G Suite OAuth 2.0 + IMAP4 Protocol in Background Service (Service Account)
VC++ -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + IMAP4 Protocol from Hotmail/Outlook Account
VC++ -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS/IMAP4 Protocol from Office 365 Account
VC++ -Retrieve Email using Microsoft OAuth 2.0 (Modern Authentication) + EWS Protocol from Office 365 in Background Service