You can send email from C# using MailMessage class. In this example i have explained how to send email using Gmail SMTP server.

You can also check " Send Email with Attachments" and " Send Email by requesting delivery/read receipt from C#"

Please find below C# source code, that shows how to send an email from a Gmail address using SMTP server. The Gmail SMTP server name is smtp.gmail.com and the port using send mail is 587 and also using NetworkCredential for password based authentication.
I have created sample windows application and added button = btnSendEmailUsingGmail, in this button click event the below code added.

using System;
using System.Net.Mail;
using System.Windows.Forms;

namespace SampleWindowsApp
{
    /// <summary>
    /// Send Email Form 
    /// </summary>
    /// <seealso cref="System.Windows.Forms.Form" />
    public partial class SendEmailForm : Form
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="SendEmailForm"/> class.
        /// </summary>
        public SendEmailForm()
        {
            InitializeComponent();
        }        

        /// <summary>
        /// Handles the Click event of the btnSendEmailUsingGmail control.
        /// Author = Himasagar Kutikuppala
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The <see cref="EventArgs"/> instance containing the event data.</param>
        private void btnSendEmailUsingGmail_Click(object sender, EventArgs e)
        {
            try
            {
                MailMessage mail = new MailMessage();
                SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");

                //Change below gmail address & To email address
                mail.From = new MailAddress("Your_Gmail_Address@gmail.com");
                mail.To.Add("To_Email_Address");
                mail.Subject = "Test Mail From Gmail";
                mail.Body = "This is for testing SMTP mail from GMAIL";

                SmtpServer.Port = 587;
                //Change your gmail user id & password
                SmtpServer.Credentials = new System.Net.NetworkCredential("username", "password");
                SmtpServer.EnableSsl = true;

                SmtpServer.Send(mail);
                MessageBox.Show("The email has been sent successfully.");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }
        }
    }
}