public static string GetXOAuthKeyStatic(
	string email,
	string accessToken
)

Public Shared Function GetXOAuthKeyStatic ( 
	email As String,
	accessToken As String
) As String

using System;
using System.Security.Cryptography.X509Certificates;
using Google.Apis.Auth.OAuth2;
using MailBee;
using MailBee.ImapMail;

public class Program
{
    public static void Main(string[] args)
    {
        String serviceAccountEmail = "88...-...7j@developer.gserviceaccount.com";
        string userEmail = "user@some-googleapps-domain.com";

        var certificate = new X509Certificate2(@"C:\Temp\a...0-privatekey.p12",
            "secret", X509KeyStorageFlags.Exportable);

        ServiceAccountCredential credential = new ServiceAccountCredential(
            new ServiceAccountCredential.Initializer(serviceAccountEmail)
        {
            User = userEmail,
            Scopes = new[] { "https://mail.google.com/" }
        }.FromCertificate(certificate));

        if (credential.RequestAccessTokenAsync(System.Threading.CancellationToken.None).Result)
        {
            string imapXOAuthKey = OAuth2.GetXOAuthKeyStatic(userEmail, credential.Token.AccessToken);

            Imap imp = new Imap();

            // Logging is not necessary but useful for debugging.
            imp.Log.Enabled = true;
            imp.Log.Filename = @"C:\Temp\log.txt";
            imp.Log.Clear();

            imp.Connect("imap.gmail.com");
            imp.Login(userEmail, imapXOAuthKey, AuthenticationMethods.SaslOAuth2,
                AuthenticationOptions.None, null);
            imp.SelectFolder("Inbox");
            imp.Disconnect();
        }
    }
}

Imports System.Security.Cryptography.X509Certificates
Imports Google.Apis.Auth.OAuth2
Imports MailBee
Imports MailBee.ImapMail

Public Class Program
    Public Shared Sub Main(args As String())
        Dim serviceAccountEmail As String = "88...-...7j@developer.gserviceaccount.com"
        Dim userEmail As String = "user@some-googleapps-domain.com"

        Dim certificate = New X509Certificate2("C:\Temp\a...0-privatekey.p12", _
            "secret", X509KeyStorageFlags.Exportable)

        Dim credential As New ServiceAccountCredential(
            New ServiceAccountCredential.Initializer(serviceAccountEmail) With { _
            .User = userEmail, _
            .Scopes = New String() {"https://mail.google.com/"} _
        }.FromCertificate(certificate))

        If credential.RequestAccessTokenAsync(System.Threading.CancellationToken.None).Result Then
            Dim imapXOAuthKey As String = OAuth2.GetXOAuthKeyStatic(userEmail, credential.Token.AccessToken)

            Dim imp As New Imap()

            ' Logging is not necessary but useful for debugging.
            imp.Log.Enabled = True
            imp.Log.Filename = "C:\Temp\log.txt"
            imp.Log.Clear()

            imp.Connect("imap.gmail.com")
            imp.Login(userEmail, imapXOAuthKey, AuthenticationMethods.SaslOAuth2, _
                AuthenticationOptions.None, Nothing)
            imp.SelectFolder("Inbox")
            imp.Disconnect()
        End If
    End Sub
End Class

using System;
using Google.Apis.Auth.OAuth2;
using MailBee;
using MailBee.ImapMail;
using MailBee.SmtpMail;

public class Program
{
    public static void Main(string[] args)
    {
        string userEmail = "john.doe@gmail.com";

        UserCredential credential;
        credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
            new ClientSecrets
            {
                ClientId = "somewhat like 628470012345.apps.googleusercontent.com",
                ClientSecret = "somewhat like 8IbRSofadFQ7ThjClxp8GBa2"
            },
            new[] { "https://mail.google.com/" },
            "user",
            System.Threading.CancellationToken.None).Result;

        string xoauthKey = OAuth2.GetXOAuthKeyStatic(userEmail, credential.Token.AccessToken);

        bool useImap = true; // Set to false to use SMTP instead of IMAP.

        if (useImap)
        {
            Imap imp = new Imap();

            // Logging is not necessary but useful for debugging.
            imp.Log.Enabled = true;
            imp.Log.Filename = @"C:\Temp\log.txt";
            imp.Log.Clear();

            imp.Connect("imap.gmail.com");
            imp.Login(userEmail, xoauthKey, AuthenticationMethods.SaslOAuth2,
                AuthenticationOptions.None, null);
            imp.SelectFolder("Inbox");
            imp.Disconnect();
            Console.WriteLine("SUCCEEDED");
        }
        else
        {
            Smtp mailer = new Smtp();
            mailer.SmtpServers.Add("smtp.gmail.com", userEmail, xoauthKey, AuthenticationMethods.SaslOAuth2);

            // Logging is not necessary but useful for debugging.
            mailer.Log.Enabled = true;
            mailer.Log.Filename = @"C:\Temp\log.txt";
            mailer.Log.Clear();

            mailer.From.Email = userEmail;
            mailer.To.Add(userEmail);
            mailer.Subject = "empty email to myself";

            mailer.Send();
        }
    }
}

Imports System.Security.Cryptography.X509Certificates
Imports Google.Apis.Auth.OAuth2
Imports MailBee
Imports MailBee.ImapMail
Imports MailBee.SmtpMail

Public Class Program
    Public Shared Sub Main(args As String())
        Dim userEmail As String = "john.doe@gmail.com"

        Dim credential As UserCredential
        credential = GoogleWebAuthorizationBroker.AuthorizeAsync(New ClientSecrets() With { _
                .ClientId = "somewhat like 628470012345.apps.googleusercontent.com", _
                .ClientSecret = "somewhat like 8IbRSofadFQ7ThjClxp8GBa2" _
            }, New String() {"https://mail.google.com/"}, "user", _
            System.Threading.CancellationToken.None).Result

        Dim xoauthKey As String = OAuth2.GetXOAuthKeyStatic(userEmail, credential.Token.AccessToken)

        Dim useImap As Boolean = True ' Set to false to use SMTP instead of IMAP.
        If useImap Then
            Dim imp As New Imap()

            ' Logging is not necessary but useful for debugging.
            imp.Log.Enabled = True
            imp.Log.Filename = "C:\Temp\log.txt"
            imp.Log.Clear()

            imp.Connect("imap.gmail.com")
            imp.Login(userEmail, xoauthKey, AuthenticationMethods.SaslOAuth2, _
                AuthenticationOptions.None, Nothing)
            imp.SelectFolder("Inbox")
            imp.Disconnect()
            Console.WriteLine("SUCCEEDED")
        Else
            Dim mailer As New Smtp()
            mailer.SmtpServers.Add("smtp.gmail.com", userEmail, xoauthKey, _
                AuthenticationMethods.SaslOAuth2)

            ' Logging is not necessary but useful for debugging.
            mailer.Log.Enabled = True
            mailer.Log.Filename = "C:\Temp\log.txt"
            mailer.Log.Clear()

            mailer.From.Email = userEmail
            mailer.To.Add(userEmail)
            mailer.Subject = "empty email to myself"

            mailer.Send()
        End If
    End Sub
End Class