ASP.NET ile SMTP mail göndermek için aşağıda yazdığım Class’tan faydalanabilirsiniz. ASP.NET’te mail ayarları için web.config‘te ayarlama yapmanız gerekiyor.
ASP.NET için Mail.cs
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Net;
using System.Net.Mail;
using System.Text;
using System.Web;
namespace DavasWeb.Models
{
    public class Mail
    {
        public static bool SendMail(MailData MailData)
        {
            try
            {
                if (string.IsNullOrEmpty(MailData.ReceiverMails))
                {
                    return false;
                }
                MailSettings MailSettings = GetMailSettings();
                MailMessage Mail = new MailMessage()
                {
                    From = MailSettings.MailSender,
                    Subject = MailData.Subject,
                    SubjectEncoding = Encoding.UTF8,
                    Body = MailData.MailContent,
                    BodyEncoding = Encoding.UTF8,
                    IsBodyHtml = true
                };
                List<string> ReceiverList = MailData.ReceiverMails.Split(new string[] { ";" }, StringSplitOptions.RemoveEmptyEntries).ToList();
                foreach (string ReceiverMail in ReceiverList)
                    Mail.To.Add(ReceiverMail);
                if (!string.IsNullOrEmpty(MailData.CcMails))
                {
                    List<string> CcList = MailData.CcMails.Split(new string[] { ";" }, StringSplitOptions.RemoveEmptyEntries).ToList();
                    foreach (string CcMail in CcList)
                        Mail.CC.Add(CcMail);
                }
                if (!string.IsNullOrEmpty(MailData.ReplyMails))
                {
                    List<string> ReplyList = MailData.ReplyMails.Split(new string[] { ";" }, StringSplitOptions.RemoveEmptyEntries).ToList();
                    Mail.ReplyToList.Add(string.Join(",", ReplyList));
                }
                SmtpClient MailClient = new SmtpClient()
                {
                    Host = MailSettings.MailServer,
                    Port = MailSettings.MailPort,
                    EnableSsl = MailSettings.MailEnableSsl,
                    UseDefaultCredentials = false,
                    Credentials = MailSettings.MailUser,
                    DeliveryMethod = SmtpDeliveryMethod.Network
                };
                MailClient.Send(Mail);
                return true;
            }
            catch (Exception e)
            {
                return false;
            }
        }
        private static MailSettings GetMailSettings()
        {
            return new MailSettings()
            {
                MailSender = new MailAddress(ConfigurationManager.AppSettings["mailSenderMail"], ConfigurationManager.AppSettings["mailSender"], Encoding.UTF8),
                MailServer = ConfigurationManager.AppSettings["mailServer"],
                MailPort = Int32.Parse(ConfigurationManager.AppSettings["mailPort"]),
                MailEnableSsl = ConfigurationManager.AppSettings["mailEnableSsl"] == "true",
                MailUser = new NetworkCredential(ConfigurationManager.AppSettings["mailUsername"], ConfigurationManager.AppSettings["mailPassword"])
            };
        }
    }
    public class MailData
    {
        public string Subject { get; set; }
        public string MailContent { get; set; }
        public string ReceiverMails { get; set; } //Mailler noktalı virgül ile ayrılmalı.
        public string CcMails { get; set; } //Mailler noktalı virgül ile ayrılmalı.
        public string ReplyMails { get; set; } //Mailler noktalı virgül ile ayrılmalı.
    }
    partial class MailSettings
    {
        public MailAddress MailSender { get; set; }
        public string MailServer { get; set; }
        public int MailPort { get; set; }
        public bool MailEnableSsl { get; set; }
        public NetworkCredential MailUser { get; set; }
    }
}ASP.NET için web.config
<!-- Sizde appSettings varsa sadece içini almanız yeterli. -->
<appSettings>
    <!-- Mail Settings -->
    <add key="mailServer" value="sunucu" />
    <add key="mailPort" value="587" />
    <add key="mailEnableSsl" value="false" />
    <add key="mailUsername" value="kullanici_adi" />
    <add key="mailPassword" value="sifre" />
    <add key="mailSender" value="gonderen" />
    <add key="mailSenderMail" value="gonderen_mail" />
</appSettings>Bu da kodun sade hali konsol ve form uygulamalarınız için kullanabilirsiniz.
Gerekli kütüphaneler;
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Net;
using System.Net.Mail;
using System.Text;
using System.Web;MailMessage Mail = new MailMessage()
{
    From = new MailAddress("gonderen_mail", "gonderen_ad_soyad", Encoding.UTF8),
    Subject = "konu",
    SubjectEncoding = Encoding.UTF8,
    Body = "mesaj", //HTML olabilir
    BodyEncoding = Encoding.UTF8,
    IsBodyHtml = true
};
Mail.To.Add("alici_mail_1");
Mail.To.Add("alici_mail_2");
Mail.To.Add("alici_mail_3");
Mail.CC.Add("cc_mail_1");
Mail.CC.Add("cc_mail_2");
Mail.CC.Add("cc_mail_3");
Mail.ReplyToList.Add("cevap_mail_1,cevap_mail_2,cevap_mail_3");
SmtpClient MailClient = new SmtpClient()
{
    Host = "sunucu", //string
    Port = port_no, //int
    EnableSsl = ssl_var_mi, //bool
    UseDefaultCredentials = false,
    Credentials = new NetworkCredential("kullanici_adi", "sifre"),
    DeliveryMethod = SmtpDeliveryMethod.Network
};
MailClient.Send(Mail);