using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var email = "billyjoe12334567..4@t.test";

        //email = null;

        Guid userIdForEmail = Guid.NewGuid();

        if (string.IsNullOrWhiteSpace(email) || email.Length > 64 || !Regex.IsMatch(email, "^.+@.+\\..+$"))

        {

            email = $"{userIdForEmail}@onpetrozone.com";

        }

        Console.WriteLine(email);

    }

}