using System;

using System.Collections.Generic;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

        List<string> emails = new List<string>()

        {

        "john.doe@domain1.com",

        "jane_doe@domain1.com",

        "patricksmith@domain2.com",

        "duckface@domain2.com",

        "erick.brown@domain3.com",

        "steeve.burn@domain4.com",

        "plum@domain4.com",

        "feather@domain5.com"

        };

        var q = emails.GroupBy(m => m.Split('@')[1]).Select(g => new List<string>(g)).Interleave();

        foreach(var l in q)

            Console.WriteLine(l);

    }

}

​

public static class Extensions

{

    public static IEnumerable<T> Interleave<T>(this IEnumerable<IEnumerable<T>> source)

    {

        var queues = source.Select(x => new Queue<T>(x)).ToList();

        while (queues.Any(x => x.Any()))

        {

            foreach (var queue in queues.Where(x => x.Any()))

            {

                yield return queue.Dequeue();

            }

        }

    }

}