If you want to send email using your Gmail account or using Gmail's smtp server in ASP.NET application or if you don't have a working smtp server to send mails using your ASP.NET application or aspx page than sending e-mail using Gmail is best option.

You need to write code like this

First of all add below mentioned namespace in code behind of aspx page from which you want to send the mail.

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("[email protected]");
  mail.To.Add("[email protected]");
  mail.From = new MailAddress("[email protected]");
  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
       ("[email protected]","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("[email protected]")
  mail.To.Add("[email protected]")
  mail.From = New MailAddress("[email protected]")
  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
       ("[email protected]","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 [email protected] 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."

than 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
("[email protected]","YourGmailPassword");
smtp.EnableSsl = true;
smtp.Send(mail);

Hope this help!!