Using Gmail IMAP OAUTH/XOAUTH2

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 Gmail Web OAuth instead of inputting user and password directly in application. This way is more secure, but a little bit complex.

https://developers.google.com/gmail/oauth_overview?hl=en

Create your project in Google Developers Console

To use Gmail OAUTH in your application, you must create a project in Google Developers Console at first.

Create your project at https://console.developers.google.com/projectcreate.
Select your project -> APIs & Services -> Dashboard -> Credentials;
Credentials -> Create Credentials -> OAuth client ID -> Web application or Other (Desktop Application). It depends on your application type.
Input a name for your application, input your current ASP/ASP.NET URL at Authorized redirect URIs, for example: http://localhost/gmailoauth/default.aspx. (Desktop Application doesn't require this step)
Click "Create", you will get your client id and client secret:
You should set detail information for your project at "OAuth consent screen" -> "Edit App".
Finally add "https://mail.google.com/" scopes at "OAuth consent screen" -> "Edit App" -> "Scopes for Google API".

Use client id and client secret to get 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 IMAP server. You can find the example codes in EAGetMail Installation Path\Samples_{Programming language/Developer Tool} project.

You should create your client id and client secret, don't use the client id in the sample project, it is only for test purpose. If you got "This app isn't verified" information, please click "advanced" -> Go to ... for test.

Use access token to retrieve email from Gmail IMAP4 server

After you get user email address and access token, you can use the following codes to retrieve email using Gmail SASL XOAUTH2 mechanism.

Example

[Visual Basic 6.0, VBScript, Visual C++, Delphi] The following example demonstrates how to receive email using Gmail IMAP OAUTH, but it doesn't demonstrates the events and mail parsing usage. To get the full samples of EAGetMail, please refer to Samples section.

[Visual Basic 6.0]
Public Sub ReceiveMail( _
ByVal userEmail As String, _
ByVal accessToken As String)
    
    Const MailServerPop3 = 0
    Const MailServerImap4 = 1
    Const MailServerEWS = 2
    Const MailServerDAV = 3
    Const MailServerMsGraph = 4

    Const MailServerAuthLogin = 0
    Const MailServerAuthCRAM5 = 1
    Const MailServerAuthNTLM = 2
    Const MailServerAuthXOAUTH2 = 3
    
    Dim oClient As New EAGetMailObjLib.MailClient
    oClient.LicenseCode = "TryIt"
    
    Dim oServer As New EAGetMailObjLib.MailServer
    oServer.Server = "imap.gmail.com"
    oServer.User = userEmail
    oServer.Password = accessToken
    oServer.Protocol = MailServerImap4
    oServer.AuthType = MailServerAuthXOAUTH2
    
    oServer.SSLConnection = True
    oServer.Port = 993 'SSL IMAP4
    
    On Error GoTo ErrorHandle
        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


[VBScript]
Sub ReceiveMail( _
ByVal userEmail, _
ByVal accessToken)
    
    Const MailServerPop3 = 0
    Const MailServerImap4 = 1
    Const MailServerEWS = 2
    Const MailServerDAV = 3
    Const MailServerMsGraph = 4
    
    Const MailServerAuthLogin = 0
    Const MailServerAuthCRAM5 = 1
    Const MailServerAuthNTLM = 2
    Const MailServerAuthXOAUTH2 = 3
    
    Dim oClient
    Set oClient = CreateObject("EAGetMailObj.MailClient")
    oClient.LicenseCode = "TryIt"
    
    Dim oServer
    Set oServer = CreateObject("EAGetMailObj.MailServer")
    oServer.Server = "imap.gmail.com"
    oServer.User = userEmail
    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
End Sub


[Visual C++]
#include "stdafx.h"
#include <windows.h>

#include "eagetmailobj.tlh"
using namespace EAGetMailObjLib;

void ReceiveMail(
        LPCTSTR userEmail, 
        LPCTSTR accessToken)
{
    ::CoInitialize(NULL);

    const int MailServerPop3 = 0;
    const int MailServerImap4 = 1;
    const int MailServerEWS = 2;
    const int MailServerDAV = 3;
    const int MailServerMsGraph = 4;
    
    const int MailServerAuthLogin = 0;
    const int MailServerAuthCRAM5 = 1;
    const int MailServerAuthNTLM = 2;
    const int MailServerAuthXOAUTH2 = 3; 

    try
    {
        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 = userEmail;
        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());
    }

    ::CoUninitialize();
}


[Delphi]
const 
    MailServerPop3 = 0;
    MailServerImap4 = 1;
    MailServerEWS = 2;
    MailServerDAV = 3;

    // Auth type
    MailServerAuthLogin = 0;
    MailServerAuthCRAM5 = 1;
    MailServerAuthNTLM = 2;
    MailServerAuthXOAUTH2 = 3;

var
    Form1: TForm1;

implementation

{$R *.dfm}

procedure ReceiveMail(userEmail: WideString; accessToken: WideString);
var
    oServer: TMailServer;
    oClient: TMailClient;
    oTools: TTools;
    oMail: IMail;
    infos: IMailInfoCollection;
    oInfo: IMailInfo;
    localInbox, fileName: WideString;
    i: Integer;
begin

    try
        // 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 := userEmail;
        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

If your application is background service which doesn't support user interaction, please have a look at this topic: Use G Suite service account to do IMAP4 OAUTH.

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

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