using System;

    using System.Linq;

    using System.Collections.Generic;

    public class Program

    {

        public static void Main()

        {

            var seed = new List<string>()

            {

                "1@a.com",

                "2@a.com",

                "3@a.com",

                "4@a.com",

                "5@a.com",

                "6@a.com",

                "7@a.com",

                "8@a.com",

                "9@a.com",

                "10@a.com",

                "1@b.com",

                "2@b.com",

                "3@b.com",

                "1@c.com",

                "4@b.com",

                "2@c.com",

                "3@c.com",

                "4@c.com"

            };

            var work = seed

                .Select(s => new EmailAddress(s)) // s.ToLowerCase() ?

                .GroupBy(s => s.Domain)

                .Select(g => new EmailAddressGroup(g))

                .ToList();

            var currentDomain = string.Empty;

            while(work.Count > 0)

            {

                var noDups = work.Where(w => w.Domain != currentDomain);

                if (noDups.Count() == 0)

                {

                    break;

                }

                var workGroup = noDups.First(w => w.Count() == noDups.Max(g => g.Count()));

                var workItem = workGroup.Remove();

                if (workGroup.Count() == 0)

                {

                    work.Remove(workGroup);

                    Console.WriteLine("removed: " + workGroup.Domain);

                }

                Console.WriteLine(workItem.FullEmail);

                currentDomain = workItem.Domain;

            }

            Console.WriteLine("Cannot disperse email addresses affectively, left overs:");

            foreach(var workGroup in work)

            {

                while(workGroup.Count() > 0)

                {

                    var item = workGroup.Remove();

                    Console.WriteLine(item.FullEmail);

                }

            }

        }

        public class EmailAddress

        {

            public EmailAddress(string emailAddress)

            {

                // Additional Email Address Validation

                var result = emailAddress.Split(new char[] {'@'}, StringSplitOptions.RemoveEmptyEntries)

                    .ToList();

                if (result.Count() != 2)

                {

                    new ArgumentException("emailAddress");

                }

                this.FullEmail = emailAddress;

                this.Name = result[0];

                this.Domain = result[1];

            }

            public string Name { get; private set; }

            public string Domain { get; private set; }

            public string FullEmail { get; private set; }

        }

        public class EmailAddressGroup

        {

            private List<EmailAddress> _emails;

            public EmailAddressGroup(IEnumerable<EmailAddress> emails)

            {

                this._emails = emails.ToList();

                this.Domain = emails.First().Domain;

            }

            public int Count()

            {

                return _emails.Count();

            }

            public string Domain { get; private set; }

            public EmailAddress Remove()

            {

                var result = _emails.First();

                _emails.Remove(result);

                return result;

            }

        }

    }