STMP 服务器的 MailKit /Google OAUTH 抛出异常“5.5.2 语法错误，再见”认证时

Question

我正在尝试使用 oAuth2 通过 Google Smtp 服务器进行身份验证。

当我尝试与服务器进行身份验证时出现此错误： MailKit.Security.AuthenticationException: '555: 5.5.2 语法错误，再见。 g2-20020a056e021e0200b002c82a3f69cesm5461647ila.8 - gsmtp'

我看到其他人在尝试发送邮件时遇到此错误，但我还没有走到这一步。以下示例应用程序可以重现该错误。

在 Google 开发者控制台上，我创建了一个项目。我创建了一个服务帐户和一个密钥。我已启用 GMAIL API。

基本身份验证有效，但我正在准备让谷歌禁用基本身份验证。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Google.Apis.Auth.OAuth2;
using MailKit;
using MailKit.Net.Smtp;
using MailKit.Security;

namespace ConsoleAppToTestMail
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            SendMail();
        }

        private static void SendMail()
        {
            try
            {
                // service account email address
                const string serviceAccount = "[email protected]";

                // import service account key p12 certificate.
                var certificate = new X509Certificate2("C:\\Certs\\smtp-support-60485-58c02993074a.p12",
                    "notasecret", X509KeyStorageFlags.Exportable);

                // G Suite user email address
                var gsuiteUser = "[email protected]";

                var serviceAccountCredentialInitializer = new ServiceAccountCredential.Initializer(serviceAccount)
                {
                    User = gsuiteUser,
                    Scopes = new[]
                    {
                        "../auth/gmail.send",
                        "https://mail.google.com/"
                    }

                }.FromCertificate(certificate);
                var credential = new ServiceAccountCredential(serviceAccountCredentialInitializer);
                // request access token
                var task = credential.RequestAccessTokenAsync(CancellationToken.None);
                var success = task.Result;
                if (!success)
                    throw new InvalidOperationException("Access token failed.");



                var oauth2 = new SaslMechanismOAuth2(credential.User, credential.Token.AccessToken);
                //var oauthBearer = new SaslMechanismOAuthBearer(credential.User, credential.Token.AccessToken);

                var smtpClient = new SmtpClient(new ProtocolLogger("google.smtp.log"));
                smtpClient.Connect("smtp.gmail.com", 587, SecureSocketOptions.Auto);

                smtpClient.Authenticate(oauth2);  // this is where the error occurs

                // send mail goes here

            }
            catch (Exception ep)
            {
                Console.WriteLine(ep.ToString());
            }

        }
    }
}

我取回的身份验证令牌是 1024 个字符。但以一长串句点结尾

ya29.c.b0AXv0zTNEf9xphDdsW_a-NtVGzk63Gj4vf4zaWyG3f-GLGOk82BREueuG41EmcjNZTo-RZxU6UWHFra2kDqv9estaLRuTDnf8yBHPYiLH2-lFYlEQcw2sxrx 0BMjL3cB01IzTII5XJR6mcQxeoYLAteB_IWovLBWnYreGIiCH3PTDos4gBFSACb1KNQ4wJqomlivoBefLulE1OHyaOesCqWonkaBcQw................................. ...................................................... ...................................................... ...................................................... ...................................................... ...................................................... ...... ...................................................... ...................................................... ...................................................... ...................................................... ...................................................... ...................................................... ...................................................... ...................................................... ...................................................... ...................................................... …………

我也尝试使用 OAuthBearer 得到相同的结果。

var oauthBearer = new SaslMechanismOAuthBearer(credential.User, credential.Token.AccessToken);