Sending Email fails in ASP.Net

i have working on site before  years on server . mail function is working fine .

but some days before my function is not working suddnely ..on new ragistration etc.

when we discuss with host side then he say reconfig mail function ..]

whats the main problem it and how to resolve it ..(mail reconfig)..

My Mail Function Code In Class File its Used In While Site ...

public void sendmessage(string message,string subject,string ToAddress)
    {   
        string FromAddress = "info@test.com";
        MailMessage MailBody = new MailMessage();
        MailBody.IsBodyHtml = true;
        MailBody.To.Add(ToAddress);
        MailBody.From = new MailAddress(FromAddress, "Test.Com", System.Text.Encoding.UTF8);
        MailBody.Subject = subject;
        MailBody.SubjectEncoding = System.Text.Encoding.UTF8;
        MailBody.Body = message;
        MailBody.BodyEncoding = System.Text.Encoding.UTF8;
        MailBody.Priority = MailPriority.Normal;
        SmtpClient SMTPHOST = new SmtpClient();
        SMTPHOST.EnableSsl = false;
        SMTPHOST.UseDefaultCredentials = true;
        SMTPHOST.Credentials = new NetworkCredential(FromAddress, "87621");
        SMTPHOST.Host = "mail.test.com";
        SMTPHOST.Send(MailBody);
    }

Thanks In Advanse ..