the file can't be deleted because it's used by another process

try
{
    MailMessage mail = new MailMessage();
    SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
    mail.From = new MailAddress("your_email_address@gmail.com");
    mail.To.Add("to_address");
    mail.Subject = "Test Mail - 1";
    mail.Body = "mail with attachment";

    System.Net.Mail.Attachment attachment;
    attachment = new System.Net.Mail.Attachment("you attachment file");
    mail.Attachments.Add(attachment);

    SmtpServer.Port = 587;
    SmtpServer.Credentials = new System.Net.NetworkCredential("username", "password");
    SmtpServer.EnableSsl = true;

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

SmtpServer.Send(mail);

attachment.Dispose();