using System;

using System.Net.Mail;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<string> lst = new List<string>();

        lst.Add("abc@man.com");

        lst.Add("abc@pioneerlogix.com");

        lst.Add("hannan.com");

        var str = GetCommaSeparatedValidEmails(lst);

        Console.WriteLine(str);

        Console.WriteLine("Hello World");

    }

    public static string GetCommaSeparatedValidEmails(List<string> listContacts)

        {

            string emailsToReturn = "";

            foreach (var contact in listContacts)

            {

                if (!string.IsNullOrEmpty(contact) && !string.IsNullOrWhiteSpace(contact))

                {

                    try

                    {

                        if (!string.IsNullOrEmpty(emailsToReturn) && !string.IsNullOrWhiteSpace(emailsToReturn))

                        {

                            emailsToReturn += ",";

                        }

                        MailAddress m = new MailAddress(contact);

                        emailsToReturn += contact;

                    }

                    catch (Exception E)

                    {

                        Console.WriteLine(contact + "==>>" + E.ToString());

                    }

                }

            }

            return emailsToReturn;

        }

}