Mail Sending Problem

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webservice

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.Web.Mail;
using System.Net;

MailMessage mail = new MailMessage();
        mail.To = "kishorethoppil@gmail.com";
        mail.Cc = "gisol@rediffmail.com";
        mail.From = "anilapv@gmail.com";
        mail.Subject = "gisol@rediffmail.com";
        mail.Body = "gisol@rediffmail.com";
        mail.BodyFormat = MailFormat.Text;
        SmtpMail.SmtpServer = "server34.hosthat.com";
        SmtpMail.Send(mail);

this is the code i used to send a mail...
SmtpMail.SmtpServer = "server34.hosthat.com"; is my office server


bt it shows an error

The server rejected one or more recipient addresses. The server response was: 503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.

please give me a solution to repair this

Kishore R
Gis Solutions