Send Email Using Gmail in ASP.NET

using System.Net.Mail;

Now write this code in click event of button

C# Code 

protected void Button1_Click(object sender, EventArgs e)

{

  MailMessage mail = new MailMessage();

  mail.To.Add("shodecode.khi@gmail.com");

  mail.To.Add("code_shode_online@yahoo.com");

  mail.From = new MailAddress("shodecode.khi@gmail.com");

  mail.Subject = "Email using Gmail";

  string Body = "Hi, this mail is to test

          sending mail"+

                "using Gmail in ASP.NET";

  mail.Body = Body;

  mail.IsBodyHtml = true;

  SmtpClient smtp = new SmtpClient();

  smtp.Host = "smtp.gmail.com"; //Or Your SMTP Server Address

  smtp.Credentials = new System.Net.NetworkCredential

       ("YourUserName@gmail.com","YourGmailPassword");

//Or

    your Smtp Email ID and Password

  smtp.EnableSsl = true;

  smtp.Send(mail);

}

VB.NET Code 

Imports System.Net.Mail
 
Protected  Sub Button1_Click
(ByVal sender As Object, ByVal e As EventArgs)
  Dim mail As MailMessage =  New MailMessage() 
  mail.To.Add("shodecode.khi@gmail.com")
  mail.To.Add("code_shode_online@yahoo.com")
  mail.From = New MailAddress("shodecode.khi@gmail.com")
  mail.Subject = "Email using Gmail"
 
  String Body = "Hi, this mail is to test
          sending mail"+ 
                "using Gmail in ASP.NET"
  mail.Body = Body
 
  mail.IsBodyHtml = True
  Dim smtp As SmtpClient =  New SmtpClient() 
  smtp.Host = "smtp.gmail.com" //Or Your SMTP Server Address
  smtp.Credentials = New System.Net.NetworkCredential
       ("YourUserName@gmail.com","YourGmailPassword")
  smtp.EnableSsl = True
  smtp.Send(mail)
End Sub

You also need to enable POP by going to settings > Forwarding and POP in your gmail
account

Change YourUserName@gmail.com to your gmail ID and YourGmailPassword to Your password
for Gmail account and test the code.

If your are getting error mentioned below 

"The SMTP server requires a secure connection or the client was not authenticated. The
server response was: 5.5.1 Authentication Required."

then you need to check your Gmail username and password.

If you are behind proxy Server then you need to write below mentioned code in your
web.config file.

<system.net>

<defaultProxy>

<proxy proxyaddress="YourProxyIpAddress"/>

defaultProxy>

system.net>

If you are still having problems them try changing port number to 587 


smtp.Host = "smtp.gmail.com,587";


If you still having problems then try changing code as mentioned below:

SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.UseDefaultCredentials = False;
smtp.Credentials = new System.Net.NetworkCredential
("YourUserName@gmail.com","YourGmailPassword");
smtp.EnableSsl = true;
smtp.Send(mail);