how to get rid of the error in sending mail?

I guess the error is because of the port
try with both 465 and 587. and make sure the username/pwd is correct and SSL is enabled. it works for me on 587
Try this
please convert it to c#

Imports System.Net
Imports System.Net.Mail

    Protected Sub cmd_send_Click(sender As Object, e As System.EventArgs) Handles cmd_send.Click
        Try
            Dim smtpserver As New SmtpClient
            Dim mailmsg As New MailMessage
            smtpserver.Host = "smtp.gmail.com"
            smtpserver.Port = 465
            smtpserver.EnableSsl = True
            smtpserver.UseDefaultCredentials = False
            mailmsg.From = New MailAddress(shivani.sharma.corporate@gmail.com)
            mailmsg.Subject = "Hi there"
            mailmsg.To.Add(shivani.sharma.corporate@gmail.com)
            mailmsg.IsBodyHtml = True
            mailmsg.Body =  "Hey there <br /> Check out http://www.ossagho.com";
            smtpserver.Send(mailmsg)
            lbl_err.Text = "Sent"
        Catch ex As Exception
            lbl_err.Text = ex.Message.ToString
        End Try
      
    End Sub

just give a check on the below link as well of gmail smtp settings
http://support.google.com/mail/bin/answer.py?hl=en&answer=78775[^]

Code written below worked for me..

#region using
using System;
using System.Diagnostics;
using System.Net.Mail; 
#endregion
 
namespace ProjectUtils
{
    /// <summary>
    /// Helps to send mail messages.
    /// </summary>
    public class EmailHelper
    {
        /// <summary>
        /// Sends the mail to recepients.
        /// </summary>
        /// <param name="from">Sender</param>
        /// <param name="password">Password of mail sender</param>
        /// <param name="to">Recepient</param>
        /// <param name="subject">Subject of Message</param>
        /// <param name="body">Message body</param>
        /// <param name="smtpClient">Name of smtp client</param>
        /// <param name="port">Port number of smtp client</param>
        /// <returns>True if succeeds, false if exception occurs</returns>
        public bool SendMail(string from, string password, string to, string subject, string body, System.Net.Mail.SmtpClient smtpClient, int port)
        {
            try
            {
            System.Net.NetworkCredential credential = new System.Net.NetworkCredential(from, password);
            System.Net.Mail.MailMessage mailMessage = new MailMessage();
            mailMessage.To.Add(to);
            mailMessage.Subject = subject;
 
            mailMessage.From = new System.Net.Mail.MailAddress(from);
            mailMessage.Body = body;
 
            System.Net.Mail.SmtpClient smtpClient = smtpClient;
            smtpClient.UseDefaultCredentials = false;
            smtpClient.EnableSsl = true;
            smtpClient.Credentials = credential;
            smtpClient.Port = port;
           
                smtpClient.Send(mailMessage);
                return true;
            }
            catch (Exception ex)
            {
                Trace.WriteLine("Exceeption in sending mail:" + ex.Message);
                return false;
            }
        }
    }

Value of smtpClient=new SmtpClient("smtp.gmail.com", 587)
port = 587
from: XXX@gmail.com
password: Password of user XXX.
Make sure that you are connected to internet otherwise you'll get Failure sending mail error as you mentioned.

Try with below code ..it worked for me.

public void SendMail(string to, string subject, string body)
{
string uname = WebConfigurationManager.AppSettings["name"].ToString();
string pwd = WebConfigurationManager.AppSettings["pwd"].ToString();
int port = Convert.ToInt32(WebConfigurationManager.AppSettings["clientport"].ToString());
string host = WebConfigurationManager.AppSettings["SMTPMailServer"].ToString();
MailMessage message = new MailMessage();
message.To.Add(new MailAddress(to));
message.From = new MailAddress(uname);
message.Subject = subject;
message.Body = body;
message.BodyEncoding = System.Text.Encoding.UTF8;
message.SubjectEncoding = System.Text.Encoding.UTF8;
SmtpClient client = new SmtpClient();
client.Port = port;
client.Host = host;
System.Net.NetworkCredential nc = new System.Net.NetworkCredential(uname, pwd);
client.EnableSsl = true;
client.UseDefaultCredentials = false;
client.Credentials = nc;
message.IsBodyHtml = true;
ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(ValidateServerCertificate);
client.Send(message);
}

public bool ValidateServerCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
// replace with proper validation
if (sslPolicyErrors == SslPolicyErrors.None)
return true;
else
return false;
}

and in web.config file set username,password and host like..

<add key="SMTPMailServer" value="smtp.gmail.com"/>
   
    <add key="name" value="ur mail id"/>
    <add key="pwd" value="set password here"/>
    <add key="clientport" value="587"/>
    <add key="EnableSSLforMail" value="true"/>