MailMessage msg;
            try
            {
                smtpclient = new SmtpClient("smtp服务器地址");
                smtpclient.UseDefaultCredentials = true;
                smtpclient.Credentials = new NetworkCredential("登陆smtp服务器的用户名", "登陆smtp服务器的密码");
                smtpclient.DeliveryMethod = SmtpDeliveryMethod.Network;

                msg = new MailMessage("发送邮件地址","接受邮件地址","邮件标题", "邮件内容");
                //附件判断
                if (tboxAttachment.Text.Length != 0)
                {
                    attachment = new Attachment(tboxAttachment.Text);
                    msg.Attachments.Add(attachment);
                }
                msg.BodyEncoding = Encoding.UTF8;//邮件编码格式
                msg.IsBodyHtml = false;//邮件内容是否HTML
                smtpclient.Send(msg);//发送邮件
            }
            catch (Exception err)
            {
                //这里写异常处理代码，比如记录异常到数据库什么的
            }