using System;

using System.Net.Mail;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main(string[] args)

    {

        var email1 = "test@test.com";

        var email2 = "test2@yea.test.co.uk";

        var email1Host = new MailAddress(email1).Host;

        var email2Host = new MailAddress(email2).Host;

        Regex regex = new Regex(@"[^.]*\.[^.]{2,3}(?:\.[^.]{2,3})?$");

        Match match = regex.Match(email2);

        if (match.Success)

        {

            Console.WriteLine(match.Value);

        }

    }

}

​