ASP.net 中的 SendEmail 显示语法错误，命令无法识别。服务器响应是：Dovecot 就绪

我想使用 ASP.NET 发送邮件，代码如下：

 public void Semail(string subject, string messageBody, string toAddress)
    {
        MailMessage mail = new MailMessage();
        mail.To.Add(toAddress);
        //mail.To.Add("[email protected]");
        mail.From = new MailAddress("[email protected]");
        mail.Subject = subject;
        string Body = messageBody;
        mail.Body = Body;
        mail.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "sina.sharif.ir"; //Or Your SMTP Server Address

        smtp.Credentials = new System.Net.NetworkCredential
            ("[email protected]", "******");
        //Or your Smtp Email ID and Password
        smtp.EnableSsl = true;
        smtp.Send(mail);
    }

但执行后我得到了这个错误：

 Syntax error, command unrecognized. The server response was: Dovecot ready.

这是堆栈跟踪

[SmtpException: Syntax error, command unrecognized. The server response was: Dovecot ready.]
   System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint) +2176152
   System.Net.Mail.SmtpClient.Send(MailMessage message) +2188821
   Novitiate.fa.Register.btnLogin_Click(Object sender, EventArgs e) +2948
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +154
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3707

SMTP 服务器可能有问题。

尝试使用 GMAIL 设置，如果邮件正在使用 GMAIL 服务器，那么很可能您的邮件服务器存在问题。 看起来您正在使用 POP3 或 IMAP 服务器而不是 SMTP 服务器，请检查您的服务器配置

static void Main(string[] args)
        {
            var client = new SmtpClient("smtp.gmail.com", 587)
            {
                Credentials = new NetworkCredential("[email protected]", "mypwd"),
                EnableSsl = true
            };
            client.Send("[email protected]", "[email protected]", "test", "testbody");
            Console.WriteLine("Sent");
            Console.ReadLine();
        }