<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <startup> 
        <supportedRuntime version="vxxx" sku=".NETFramework,Version=vxxx"/>
    </startup>
  <connectionStrings>
    <add name="ConnString" connectionString="Data Source=xxx;Initial Catalog=xxx;User ID=xxx;Password=xxx" providerName="xxx"/>
  </connectionStrings>
</configuration>

using SendEmail.Modules.SendEmails;
using System.ServiceProcess;
using System.Timers;

namespace SendEmail
{
    public partial class OrderService : ServiceBase
    {
        Send email = new Send();
        Timer time = new Timer(5000);

        public OrderService()
        {
            InitializeComponent();
        }

        protected override void OnStart(string[] args)
        {
　　　　　　　#region 服务开始，设置定时轮询
//时间间隔（毫秒）
            //Timer time = new Timer(5000);
            //保持持续执行
            time.AutoReset = true;
            time.Elapsed += new System.Timers.ElapsedEventHandler(KeepRunEvent);
            time.Start();
　　　　
　　　　　　　#endregion

        }
        private void KeepRunEvent(object sender, System.Timers.ElapsedEventArgs e)
        {
            #region 避免上次任务尚未完成，下一次任务又开始了

            time.Enabled = false;
            email.SendService();
            time.Enabled = true;

　　　　　　  #endregion
        }
        protected override void OnStop()
        {
　　　　　    #region 服务停止
　　　　　　　 #endregon
        }
    }
}

using SendEmail.Modules.ConnectDB;
using System;
using System.Data;
using System.Net.Mail;
using System.Text;

namespace SendEmail.Modules.SendEmails
{
    public class Send
    {
        DBHelper bll = new DBHelper();
        public void SendService()
        {
            string sql = "";//查找未删除，未发送，未锁定数据
            DataTable dt = bll.Select("select * from xxx where xxx=0 and xxx=0 and xx=0");
            if (dt.Rows.Count > 0)
            {
                for (int i = 0; i < dt.Rows.Count; i++)
                {
                    //接收人
                    string accepter = dt.Rows[i]["xxx"].ToString();
                    //邮件内容
                    string content = dt.Rows[i]["xxx"].ToString();
                    if (!string.IsNullOrEmpty(accepter) && !string.IsNullOrEmpty(content))
                    {

                        MailMessage msg = new MailMessage();
                        msg.To.Add(accepter);//收件人地址 
                        msg.From = new MailAddress("xxx", "个人网站");//发件人邮箱，名称 
                        msg.Subject = "来自个人网站的邮件";//邮件标题 
                        msg.SubjectEncoding = Encoding.UTF8;//标题格式为UTF8 
                        msg.Body = content;//邮件内容 
                        msg.BodyEncoding = Encoding.UTF8;//内容格式为UTF8 

                        SmtpClient client = new SmtpClient();
                        client.Host = "smtp.qq.com";//SMTP服务器地址 
                        client.Port = 587;//SMTP端口，QQ邮箱填写587 
　　　　　　　　　　　　　　
　　　　　　　　　　　　　　/**********下面顺序不要反**********/
　　　　　　　　　　　　　　client.EnableSsl = true;//启用SSL加密 
                        client.UseDefaultCredentials = false;
                        //发件人邮箱账号，授权码
                        client.Credentials = new System.Net.NetworkCredential("xxx", "xxx");

                        try
                        {
                            client.Send(msg);//发送邮件
                            sql = "update xxx set xxx=1,xxx=‘success‘,xxx=‘发送成功‘ where xxx=" + dt.Rows[i]["xxx"].ToString();
                            bll.RunSql(sql);
                        }
                        catch (Exception e)
                        {
                            //return false;
                            sql = "update xxx set xxx=1,xxx=‘error‘,xxx=‘发送失败，" + e.Message + "‘ where xxx=" + dt.Rows[i]["xxx"].ToString();
                            bll.RunSql(sql);
                        }
                    }
                    else
                    {
                        sql = "update xxx set xxx=1,xxx=‘accident‘,xxx=‘接收人或发送内容为空‘ where xxx=" + dt.Rows[i]["xxx"].ToString();
                        bll.RunSql(sql);
                    }
                }
            }
        }
    }
}