public static void SendMail(string recipient, string subject, string body, string attachmentFilename)
{
    SmtpClient smtpClient = new SmtpClient();
    NetworkCredential basicCredential = new NetworkCredential(MailConst.Username, MailConst.Password);
    MailMessage message = new MailMessage();
    MailAddress fromAddress = new MailAddress(MailConst.Username);

    // setup up the host, increase the timeout to 5 minutes
    smtpClient.Host = MailConst.SmtpServer;
    smtpClient.UseDefaultCredentials = false;
    smtpClient.Credentials = basicCredential;
    smtpClient.Timeout = (60 * 5 * 1000);

    message.From = fromAddress;
    message.Subject = subject;
    message.IsBodyHtml = false;
    message.Body = body;
    message.To.Add(recipient);

    if (attachmentFilename != null)
        message.Attachments.Add(new Attachment(attachmentFilename));

    smtpClient.Send(message);
}

if (attachmentFilename != null)
{
    Attachment attachment = new Attachment(attachmentFilename, MediaTypeNames.Application.Octet);
    ContentDisposition disposition = attachment.ContentDisposition;
    disposition.CreationDate = File.GetCreationTime(attachmentFilename);
    disposition.ModificationDate = File.GetLastWriteTime(attachmentFilename);
    disposition.ReadDate = File.GetLastAccessTime(attachmentFilename);
    disposition.FileName = Path.GetFileName(attachmentFilename);
    disposition.Size = new FileInfo(attachmentFilename).Length;
    disposition.DispositionType = DispositionTypeNames.Attachment;
    message.Attachments.Add(attachment);
}

using System.Net;
using System.Net.Mail;

public void email_send()
{
    MailMessage mail = new MailMessage();
    SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
    mail.From = new MailAddress("your mail@gmail.com");
    mail.To.Add("to_mail@gmail.com");
    mail.Subject = "Test Mail - 1";
    mail.Body = "mail with attachment";

    System.Net.Mail.Attachment attachment;
    attachment = new System.Net.Mail.Attachment("c:/textfile.txt");
    mail.Attachments.Add(attachment);

    SmtpServer.Port = 587;
    SmtpServer.Credentials = new System.Net.NetworkCredential("your mail@gmail.com", "your password");
    SmtpServer.EnableSsl = true;

    SmtpServer.Send(mail);

}

try
{
    SmtpClient mailServer = new SmtpClient("smtp.gmail.com", 587);
    mailServer.EnableSsl = true;

    mailServer.Credentials = new System.Net.NetworkCredential("myemail@gmail.com", "mypassword");

    string from = "myemail@gmail.com";
    string to = "reciever@gmail.com";
    MailMessage msg = new MailMessage(from, to);
    msg.Subject = "Enter the subject here";
    msg.Body = "The message goes here.";
    msg.Attachments.Add(new Attachment("D:\myfile.txt"));
    mailServer.Send(msg);
}
catch (Exception ex)
{
    Console.WriteLine("Unable to send email. Error : " + ex);
}

System.Net.Mail.Attachment attachment;
attachment = New System.Net.Mail.Attachment(Server.MapPath("~/App_Data/hello.pdf"));
mail.Attachments.Add(attachment);

private void btnSent_Click(object sender, EventArgs e)
{
    try
    {
        MailMessage mail = new MailMessage();
        SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");

        mail.From = new MailAddress(txtAcc.Text);
        mail.To.Add(txtToAdd.Text);
        mail.Subject = txtSub.Text;
        mail.Body = txtContent.Text;
        System.Net.Mail.Attachment attachment;
        attachment = new System.Net.Mail.Attachment(txtAttachment.Text);
        mail.Attachments.Add(attachment);

        SmtpServer.Port = 587;
        SmtpServer.Credentials = new System.Net.NetworkCredential(txtAcc.Text, txtPassword.Text);

        SmtpServer.EnableSsl = true;

        SmtpServer.Send(mail);
        MessageBox.Show("mail send");
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
    }
}

private void button1_Click(object sender, EventArgs e)
{
    MailMessage mail = new MailMessage();
    openFileDialog1.ShowDialog();
    System.Net.Mail.Attachment attachment;
    attachment = new System.Net.Mail.Attachment(openFileDialog1.FileName);
    mail.Attachments.Add(attachment);
    txtAttachment.Text =Convert.ToString (openFileDialog1.FileName);
}

private void btnAtt_Click(object sender, EventArgs e) {

    openFileDialog1.ShowDialog();
    Attachment myFile = new Attachment(openFileDialog1.FileName);

    MyMsg.Attachments.Add(myFile);


}

public void Send(string from, string password, string to, string Message, string subject, string host, int port, string file)
{

MailMessage email = new MailMessage();
email.From = new MailAddress(from);
email.To.Add(to);
email.Subject = subject;
email.Body = Message;
SmtpClient smtp = new SmtpClient(host, port);
smtp.UseDefaultCredentials = false;
NetworkCredential nc = new NetworkCredential(from, password);
smtp.Credentials = nc;
smtp.EnableSsl = true;
email.IsBodyHtml = true;
email.Priority = MailPriority.Normal;
email.BodyEncoding = Encoding.UTF8;

if (file.Length > 0)
{
Attachment attachment;
attachment = new Attachment(file);
email.Attachments.Add(attachment);
}

// smtp.Send(email);
smtp.SendCompleted += new SendCompletedEventHandler(SendCompletedCallBack);
string userstate = "sending ...";
smtp.SendAsync(email, userstate);
}

private static void SendCompletedCallBack(object sender,AsyncCompletedEventArgs e) {
string result = "";
if (e.Cancelled)
{

MessageBox.Show(string.Format("{0} send canceled.", e.UserState),"Message",MessageBoxButtons.OK,MessageBoxIcon.Information);
}
else if (e.Error != null)
{
MessageBox.Show(string.Format("{0} {1}", e.UserState, e.Error), "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
}
else {
MessageBox.Show("your message is sended", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
}

}

using (OpenFileDialog attachement = new OpenFileDialog()
{
Filter = "Exel Client|*.png",
ValidateNames = true
})
{
if (attachement.ShowDialog() == DialogResult.OK)
{
Send("yourmail@gmail.com", "gmail_password", "tomail@gmail.com", "just smile ", "mail with attachement", "smtp.gmail.com", 587, attachement.FileName);

}
}