I'd like to send email from my ecommerce web by using a Gmail but I don't know how to modify the code(I didn't write it myself). Thanks for your help.

This's the code :
  
 // Generic method for sending emails
    public static void SendMail(string from, string to, string subject, string body)
    {
        // Configure mail client (may need additional code for authenticated SMTP servers)
        SmtpClient mailClient = new SmtpClient(ABCConfiguration.MailServer);
        NetworkCredential nc = new NetworkCredential("abc@gmail.com", "1234");
        mailClient.Credentials = nc;
        // Create the mail message
        MailMessage mailMessage = new MailMessage("abc@gmail.com", to, subject, body);
        // Send mail
        mailClient.Send(mailMessage);
    }

In web.config

 <add key="MailServer" value="smtp.gmail.com " /> 

This's gmail configuration :

Incoming Mail (POP3) Server - requires SSL: pop.gmail.com
Use SSL: Yes
Port: 995 
Outgoing Mail (SMTP) Server - requires TLS: smtp.gmail.com (use authentication)
Use Authentication: Yes
Use STARTTLS: Yes (some clients call this SSL)
Port: 465 or 587 

you may need to make some more settings changes to your mailClient object, based on what I see in your gmail configuration (SSL and PORT)

            mailClient.Port = 995;
            mailClient.EnableSsl = true;

Thanks for your advise and I modified the code as what you told me. Unfortunately, I've got this error. Could you tell me how to fix it out?

from all the research i have done for this issue, the problem is that the connection isn't actually opening. Some searches I have done suggest that the particular port you are using might be closed off at the OS or the Firewall.  Also, the mailClient.Credentials might need something extra beyond a username/password. perhaps you can only READ emails from the GMAIL SMTP server and not actually send through this type of transport.  It's hard to say since I generally use my ISP host's SMTP server to send email (some block sending though a network not of their making, like Charter communications, to prevent people using their service as SPAM machines).

are you building a spam machine?