通过本地主机发送的电子邮件，但当我尝试从我的服务器发送时未发送

通过本地主机发送的电子邮件，但当我尝试从服务器发送时未发送。

怎么办。

注意：我使用的是port 25和host：mail.mydomain.com。

发送邮件失败。

你能帮我吗？

我使用了以下代码：

public static string SendMail(string host , string from , string pass , int port , string pTo, string subject, string body, string attach)
    {
        string Host = host;
        string ReceiverEmailAddres = pTo;
        string SenderEmailAddresss = from;
        string SenderEmailPassword = pass;
        SmtpClient MyMail = new SmtpClient();
        MailMessage MyMsg = new MailMessage();
        MyMail.Host = Host;
        MyMail.Port = port;
        MyMsg.To.Add(new MailAddress(ReceiverEmailAddres));
        MyMsg.Subject = subject;
        if (attach != string.Empty)
        {
            Attachment data = new Attachment(attach);
            MyMsg.Attachments.Add(data);
        }
        MyMsg.IsBodyHtml = true;
        MyMsg.From = new MailAddress(SenderEmailAddresss);
        MyMsg.Body = body;

        MyMail.UseDefaultCredentials = false;
        NetworkCredential MyCredentials = new NetworkCredential(SenderEmailAddresss, SenderEmailPassword);
        MyMail.Credentials = MyCredentials;
        try
        {
            MyMail.Send(MyMsg);
            return "ok";
        }
        catch(Exception ex)
        {
            return ex.Message;
        }
    }