服务器不支持安全连接。

服务器不支持安全连接，是代码的哪里出现问题呢。
public class Email
{
#region 全局变量

String AddressFrom = String.Empty; //发件地址
String AddressTo = String.Empty; //收件地址
String MailName = String.Empty; //邮件名称
String MailTitle = String.Empty; //邮件标题
String MailContent = String.Empty; //邮件内容
String SMTPServer = String.Empty; //SMTP地址
Int32 SMTPPort = 25; //SMTP端口
String SMTPUsername = String.Empty; //SMTP用户名
String SMTPPassword = String.Empty; //SMTP密码
Boolean SMTLSSL = true ; //SMTP加密形式

ReadXML ConfigEmail = new ReadXML();

#endregion

#region 公共方法

public String SendMail()
{

MailMessage Email = new MailMessage();
MailAddress MailFrom = new MailAddress(AddressFrom, MailName);
Email.From = MailFrom;
Email.To.Add(AddressTo);
Email.Subject = MailTitle;
Email.SubjectEncoding = System.Text.Encoding.GetEncoding("GB2312");
Email.Body = MailContent;
Email.BodyEncoding = System.Text.Encoding.GetEncoding("GB2312");
Email.Priority = MailPriority.Normal;
Email.IsBodyHtml = false;

SmtpClient SmtpMail = new SmtpClient(SMTPServer, SMTPPort);
SmtpMail.Credentials = new NetworkCredential(SMTPUsername, SMTPPassword);
SmtpMail.EnableSsl = SMTLSSL;
SmtpMail.DeliveryMethod = SmtpDeliveryMethod.Network;

// SmtpMail.SendCompleted += new SendCompletedEventHandler(SendCompletedCallback);

try
{
//SmtpMail.SendAsync(Email, (object)Guid.NewGuid());
SmtpMail.Send(Email);
}
catch (SmtpFailedRecipientsException ex)
{
return ex.Message;
}
catch (Exception ex)
{
return ex.Message;
}
finally
{
Email.Dispose();
}
return String.Empty;
}

public static bool sendMail(string host, string userName, string pwd, bool ssl, string from, string to, string subject, string body)
{
System.Net.Mail.SmtpClient mail = new System.Net.Mail.SmtpClient();

mail.Host = host;//smtp
mail.Credentials = new System.Net.NetworkCredential(userName, pwd);
mail.EnableSsl = ssl;//发送连接套接层是否加密 例如用gmail发是加密的
System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage(from, to);
message.Body = body;
message.Subject = subject;
message.SubjectEncoding = System.Text.Encoding.GetEncoding("gb2312");
message.BodyEncoding = System.Text.Encoding.GetEncoding("gb2312");
message.IsBodyHtml = true;
try
{
mail.Send(message);
return true;
}
catch
{
return false;
}
}