how to remove error: Mailbox unavailable. The server response was: 5.7.1 Message rejected as spam by Content Filtering

Mailbox unavailable. The server response was: 5.7.1 Message rejected as spam by Content Filtering

dt.OpenCon();
       string str = "SELECT MA_First_Name,MA_Email_Id FROM Master_Applicant WHERE Month(MA_DOB) = Month(CURRENT_TIMESTAMP) AND Day(MA_DOB) = Day(CURRENT_TIMESTAMP)";

       DataSet ds = dt.ExecuteDataSet(str);
       if (ds.Tables[0].Rows.Count > 0)
       {
           ViewState["Name"] = ds.Tables[0].Rows[0]["MA_First_Name"].ToString();
           ViewState["Email"] = ds.Tables[0].Rows[0]["MA_Email_Id"].ToString();

       }

       string name = ViewState["Name"].ToString();
       string email = ViewState["Email"].ToString();

       try
       {
           string ToEmailID = email.ToString();
           string Fromemail = "kgirish@elinktechnology.net";
           string copymail = "weblink@elinktechnology.net";
           string copymailto = "kgirish@elinktechnology.net";
           string tomailcc = "seema@elinktechnology.net";

           System.Net.Mail.MailMessage mess = new System.Net.Mail.MailMessage();
           mess.Subject = "Birthday Mail";
           mess.IsBodyHtml = true;
           mess.Body = "<table style='width:100%; font-family:Arial; color:Black; size:10px; text-align:left'>" +
           "<tr><td style='font-weight:bold'>" +
           "Happy Birthday,<br/><br/>" +
           "</td></tr>" +

           "<tr><td>" +
           "Wish You Many Many Happy Returns of the Day "+ ViewState["Name"].ToString() +" <br/><br />" +
           "</td></tr>" +
           "<p>" +
           "</p>" +
           "<tr><td> <a href='http://www.elinktechnologies.net/elm/'>Appraisal Form<td></tr>" +
           "<tr><td>" +
           "God Bless You..<br/><br/><br />" +
           "</td></tr>" +

           "<tr>" +
           "<td>" +
           "Regards," +
           "</td></tr>" +
           "<tr>" +
           "<td style='font-weight:bold'>" +
           "Elink Management Team" +
           "</td></tr>" +
           "</table>";

           mess.To.Add(ToEmailID);
           if ((copymailto.ToString() != string.Empty) && (copymailto.ToString() != null))
           {
               mess.To.Add(copymailto);
           }
           mess.From = new MailAddress(Fromemail);
           mess.CC.Add(tomailcc);
           if ((tomailcc.ToString() != string.Empty) && (tomailcc.ToString() != null))
           {
               mess.CC.Add(tomailcc);
           41}
           SmtpClient sc = new SmtpClient();
           sc.Host = "180.92.175.6";
           sc.Send(mess);

           ViewState["Email_sent"] = "Y";
           ViewState["Email_reason"] = "N";


       }
       catch(Exception ex)
       {
           ViewState["Email_Sent"] = "N";
           ViewState["Email_Reason"] = ex.Message;
       }
   }