出现这个错误是由于ASP.NET CORE应用程序中使用了OAuth2身份验证。要解决此问题，需要更改应用程序设置以允许此操作，并获取正确的OAuth2令牌。

以下是一个示例，展示如何使用Gmail帐户发送电子邮件：

1.在Startup.cs文件中，将你的电子邮件设置添加到ConfigureServices方法中：

services.AddAuthentication(options =>
{
    options.DefaultAuthenticateScheme = GoogleDefaults.AuthenticationScheme;
    options.DefaultChallengeScheme = GoogleDefaults.AuthenticationScheme;
})
.AddGoogle(options =>
{
    options.ClientId = Configuration["Authentication:Google:ClientId"];
    options.ClientSecret = Configuration["Authentication:Google:ClientSecret"];
});
services.AddTransient();

2.在应用程序中创建一个EmailSender.cs文件，在其中添加发送电子邮件的方法：

using System.Threading.Tasks;
using Google.Apis.Auth.OAuth2;
using Google.Apis.Gmail.v1;
using Google.Apis.Gmail.v1.Data;
using Google.Apis.Services;
using Google.Apis.Util.Store;
using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Authentication.Google;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;

public class EmailSender
{
    private readonly GmailService _gmailService;
    private readonly IConfiguration _configuration;
    private readonly IHostingEnvironment _hostingEnvironment;

    public EmailSender(IConfiguration configuration, IHostingEnvironment hostingEnvironment)
    {
        _configuration = configuration;
        _hostingEnvironment = hostingEnvironment;
        var credential = GoogleWebAuthorizationBroker.AuthorizeAsync(new ClientSecrets
        {
            ClientId = _configuration["Authentication:Google:ClientId"],
            ClientSecret = _configuration["Authentication:Google:ClientSecret"]
        },
        new[] { GmailService.Scope.GmailSend }, "user", CancellationToken.None, new FileDataStore(_hostingEnvironment.WebRootPath)).Result;
        _gmailService = new GmailService(new BaseClientService.Initializer()
        {
            HttpClientInitializer = credential,
            ApplicationName = _configuration["Authentication:Google:ApplicationName"],
        });
     }

    public async Task SendMessageAsync(string from, string to, string subject, string