public class EmailHelper
    {
        private MailMessage mMailMessage;   //主要处理发送邮件的内容（如：收发人地址、标题、主体、图片等等）
        private SmtpClient mSmtpClient; //主要处理用smtp方式发送此邮件的配置信息（如：邮件服务器、发送端口号、验证方式等等）
        private int mSenderPort;   //发送邮件所用的端口号（htmp协议默认为25）
        private string mSenderServerHost;    //发件箱的邮件服务器地址（IP形式或字符串形式均可）
        private string mSenderPassword;    //发件箱的密码
        private string mSenderUsername;   //发件箱的用户名（即@符号前面的字符串，例如：[email protected]，用户名为：hello）
        private bool mEnableSsl;    //是否对邮件内容进行socket层加密传输
        private bool mEnablePwdAuthentication;  //是否对发件人邮箱进行密码验证

        ///<summary>
        /// 构造函数
        ///</summary>
        ///<param name="server">发件箱的邮件服务器地址</param>
        ///<param name="toMail">收件人地址（可以是多个收件人，程序中是以“;"进行区分的）</param>
        ///<param name="fromMail">发件人地址</param>
        ///<param name="subject">邮件标题</param>
        ///<param name="emailBody">邮件内容（可以以html格式进行设计）</param>
        ///<param name="username">发件箱的用户名（即@符号前面的字符串，例如：[email protected]，用户名为：hello）</param>
        ///<param name="password">发件人邮箱密码</param>
        ///<param name="port">发送邮件所用的端口号（htmp协议默认为25）</param>
        ///<param name="sslEnable">true表示对邮件内容进行socket层加密传输，false表示不加密</param>
        ///<param name="pwdCheckEnable">true表示对发件人邮箱进行密码验证，false表示不对发件人邮箱进行密码验证</param>
        public EmailHelper(string server, string toMail, string fromMail, string subject, string emailBody, string username, string password, string port, bool sslEnable, bool pwdCheckEnable)
        {
            try
            {
                mMailMessage = new MailMessage();
                mMailMessage.To.Add(toMail);
                mMailMessage.From = new MailAddress(fromMail);
                mMailMessage.Subject = subject;
                mMailMessage.Body = emailBody;
                mMailMessage.IsBodyHtml = true;
                mMailMessage.BodyEncoding = System.Text.Encoding.UTF8;
                mMailMessage.Priority = MailPriority.Normal;
                this.mSenderServerHost = server;
                this.mSenderUsername = username;
                this.mSenderPassword = password;
                this.mSenderPort = Convert.ToInt32(port);
                this.mEnableSsl = sslEnable;
                this.mEnablePwdAuthentication = pwdCheckEnable;
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }

        ///<summary>
        /// 添加附件
        ///</summary>
        ///<param name="attachmentsPath">附件的路径集合，以分号分隔</param>
        public void AddAttachments(string attachmentsPath)
        {
            try
            {
                string[] path = attachmentsPath.Split(\';\'); //分隔符号可以自定义
                Attachment data;
                ContentDisposition disposition;
                for (int i = 0; i < path.Length; i++)
                {
                    data = new Attachment(path[i], MediaTypeNames.Application.Octet);
                    disposition = data.ContentDisposition;
                    disposition.CreationDate = File.GetCreationTime(path[i]);
                    disposition.ModificationDate = File.GetLastWriteTime(path[i]);
                    disposition.ReadDate = File.GetLastAccessTime(path[i]);
                    mMailMessage.Attachments.Add(data);
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }
        ///<summary>
        /// 添加附件
        ///</summary>
        ///<param name="stream">网络附件，上传Stream流</param>
        public void AddAttachments(Stream stream, string name)
        {
            try
            {
                //string[] path = attachmentsPath.Split(\';\'); //以什么符号分隔可以自定义
                Attachment data;
                data = new Attachment(stream, name);
                mMailMessage.Attachments.Add(data);

            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }

        ///<summary>
        /// 发送邮件
        ///</summary>
        public void Send()
        {
            try
            {
                if (mMailMessage != null)
                {
                    //建立发送对象client,验证邮件服务器，服务器端口，用户名，以及密码
                    mSmtpClient = new SmtpClient();
                    mSmtpClient.Host = this.mSenderServerHost;
                    mSmtpClient.Port = this.mSenderPort;
                    mSmtpClient.UseDefaultCredentials = false;
                    mSmtpClient.EnableSsl = this.mEnableSsl;
                    if (this.mEnablePwdAuthentication)
                    {
                        System.Net.NetworkCredential nc = new System.Net.NetworkCredential(this.mSenderUsername, this.mSenderPassword);
                        //mSmtpClient.Credentials = new System.Net.NetworkCredential(this.mSenderUsername, this.mSenderPassword);
                        //NTLM: Secure Password Authentication in Microsoft Outlook Express
                        mSmtpClient.Credentials = nc.GetCredential(mSmtpClient.Host, mSmtpClient.Port, "NTLM");
                    }
                    else
                    {
                        mSmtpClient.Credentials = new System.Net.NetworkCredential(this.mSenderUsername, this.mSenderPassword);
                    }
                    mSmtpClient.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
                    mSmtpClient.Send(mMailMessage);
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }
    }

//添加本地附件
emailHelper.AddAttachments("C:\\附件1.txt");
//发送邮件
emailHelper.Send();

string url = "https://pic.cnblogs.com/avatar/35695/20201013225735.png";
WebClient web = new WebClient();
//从网络路径下载资源作为 Byte 数组
byte[] a = web.DownloadData(url);
Stream s = new MemoryStream(a);
string name = url.Substring(url.LastIndexOf("."));
//通过Stream添加附件
emailHelper.AddAttachments(s, "博客园团队" + name);

//发送邮件
emailHelper.Send();