MailAddress to = new MailAddress("abc@gmail.com,xyz@gmail.com");

MailMessage msg = new MailMessage();
msg.Body = ....;
msg.To.Add(...);
msg.To.Add(...);

SmtpClient smtp = new SmtpClient();
smtp.Send(msg);

MailAddress to = new MailAddress(
    String.Format("{0} <{1}>",display_name, address));

using System;
using System.Net.Mail;

public class Test
{
 public static void Main()
 {
  SmtpClient client = new SmtpClient("smtphost", 25);
  MailMessage msg = new MailMessage("x@y.com", "a@b.com,c@d.com");
  msg.Subject = "sdfdsf";
  msg.Body = "sdfsdfdsfd";
  client.UseDefaultCredentials = true;
  client.Send(msg);

 }
}

msg.To.Add(new MailAddress("your@email1.com","Your name 1"));
msg.To.Add(new MailAddress("your@email2.com","Your name 2"));

//1.The ACCOUNT
            MailAddress fromAddress = new MailAddress("myaccount@myaccount.com", "my display name");
            String fromPassword = "password";

            //2.The Destination email Addresses
            MailAddressCollection TO_addressList = new MailAddressCollection();

            //3.Prepare the Destination email Addresses list
            foreach (var curr_address in mailto.Split(new [] {";"}, StringSplitOptions.RemoveEmptyEntries))
            {
                MailAddress mytoAddress = new MailAddress(curr_address, "Custom display name");
                TO_addressList.Add(mytoAddress);
            }

            //4.The Email Body Message
            String body = bodymsg;

            //5.Prepare GMAIL SMTP: with SSL on port 587
            var smtp = new SmtpClient
            {
                Host = "smtp.gmail.com",
                Port = 587,
                EnableSsl = true,
                DeliveryMethod = SmtpDeliveryMethod.Network,
                Credentials = new NetworkCredential(fromAddress.Address, fromPassword),
                Timeout = 30000
            };


            //6.Complete the message and SEND the email:
            using (var message = new MailMessage()
            {
                From = fromAddress,
                Subject = subject,
                Body = body,
            })
            {
                message.To.Add(TO_addressList.ToString());
                smtp.Send(message);
            }

namespace WebForms.Code.Logging {

    public class ObserverLogToEmail: ILog {
        private string from;
        private string to;
        private string subject;
        private string body;
        private SmtpClient smtpClient;
        private MailMessage mailMessage;
        private MailPriority mailPriority;
        private MailAddressCollection mailAddressCollection;
        private MailAddress fromMailAddress, toMailAddress;
        public MailAddressCollection toMailAddressCollection {
            get;
            set;
        }
        public MailAddressCollection ccMailAddressCollection {
            get;
            set;
        }
        public MailAddressCollection bccMailAddressCollection {
            get;
            set;
        }

        public ObserverLogToEmail(string from, string to, string subject, string body, SmtpClient smtpClient) {
            this.from = from;
            this.to = to;
            this.subject = subject;
            this.body = body;

            this.smtpClient = smtpClient;
        }

        public ObserverLogToEmail(MailAddress fromMailAddress, MailAddress toMailAddress,
        string subject, string content, SmtpClient smtpClient) {
            try {
                this.fromMailAddress = fromMailAddress;
                this.toMailAddress = toMailAddress;
                this.subject = subject;
                this.body = content;

                this.smtpClient = smtpClient;

                mailAddressCollection = new MailAddressCollection();

            } catch {
                throw new SmtpException(SmtpStatusCode.CommandNotImplemented);
            }
        }

        public ObserverLogToEmail(MailAddressCollection fromMailAddressCollection,
        MailAddressCollection toMailAddressCollection,
        string subject, string content, SmtpClient smtpClient) {
            try {
                this.toMailAddressCollection = toMailAddressCollection;
                this.ccMailAddressCollection = ccMailAddressCollection;
                this.subject = subject;
                this.body = content;

                this.smtpClient = smtpClient;

            } catch {
                throw new SmtpException(SmtpStatusCode.CommandNotImplemented);
            }
        }

        public ObserverLogToEmail(MailAddressCollection toMailAddressCollection,
        MailAddressCollection ccMailAddressCollection,
        MailAddressCollection bccMailAddressCollection,
        string subject, string content, SmtpClient smtpClient) {
            try {
                this.toMailAddressCollection = toMailAddressCollection;
                this.ccMailAddressCollection = ccMailAddressCollection;
                this.bccMailAddressCollection = bccMailAddressCollection;

                this.subject = subject;
                this.body = content;

                this.smtpClient = smtpClient;

            } catch {
                throw new SmtpException(SmtpStatusCode.CommandNotImplemented);
            }
        }#region ILog Members

        // sends a log request via email.
        // actual email 'Send' calls are commented out.
        // uncomment if you have the proper email privileges.
        public void Log(object sender, LogEventArgs e) {
            string message = "[" + e.Date.ToString() + "] " + e.SeverityString + ": " + e.Message;
            fromMailAddress = new MailAddress("", "HaNN", System.Text.Encoding.UTF8);
            toMailAddress = new MailAddress("", "XXX", System.Text.Encoding.UTF8);

            mailMessage = new MailMessage(fromMailAddress, toMailAddress);
            mailMessage.Subject = subject;
            mailMessage.Body = body;

            // commented out for now. you need privileges to send email.
            // _smtpClient.Send(from, to, subject, body);
            smtpClient.Send(mailMessage);
        }

        public void LogAllEmails(object sender, LogEventArgs e) {
            try {
                string message = "[" + e.Date.ToString() + "] " + e.SeverityString + ": " + e.Message;

                mailMessage = new MailMessage();
                mailMessage.Subject = subject;
                mailMessage.Body = body;

                foreach(MailAddress toMailAddress in toMailAddressCollection) {
                    mailMessage.To.Add(toMailAddress);

                }
                foreach(MailAddress ccMailAddress in ccMailAddressCollection) {
                    mailMessage.CC.Add(ccMailAddress);
                }
                foreach(MailAddress bccMailAddress in bccMailAddressCollection) {
                    mailMessage.Bcc.Add(bccMailAddress);
                }
                if (smtpClient == null) {
                    var smtp = new SmtpClient {
                        Host = "smtp.gmail.com",
                        Port = 587,
                        EnableSsl = true,
                        DeliveryMethod = SmtpDeliveryMethod.Network,
                        Credentials = new NetworkCredential("yourEmailAddress", "yourPassword"),
                        Timeout = 30000
                    };
                } else smtpClient.SendAsync(mailMessage, null);
            } catch (Exception) {

                throw;
            }
        }
    }

try
                {
                    string s = textBox2.Text;
                    string[] f = s.Split(',');

                    for (int i = 0; i < f.Length; i++)
                    {
                        MailMessage message = new MailMessage(); // Create instance of message
                        message.To.Add(f[i]); // Add receiver
                        message.From = new System.Net.Mail.MailAddress(c);// Set sender .In this case the same as the username
                        message.Subject = label3.Text; // Set subject
                        message.Body = richTextBox1.Text; // Set body of message
                        client.Send(message); // Send the message
                        message = null; // Clean up
                    }

                }

                catch (Exception ex)
                {

                    MessageBox.Show(ex.Message);
                }

MailAddress fromAddress = new MailAddress  (fromMail,fromName);
MailAddress toAddress = new MailAddress(toMail,toName);
MailMessage message = new MailMessage(fromAddress,toAddress);
message.Subject = subject;
message.Body = body;
SmtpClient smtp = new SmtpClient()
{
    Host = host, Port = port,
    enabHost = "smtp.gmail.com",
    Port = 25,
    EnableSsl = true,
    UseDefaultCredentials = true,
    Credentials = new  NetworkCredentials (fromMail, password)
};
smtp.send(message);