Extensions.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace EmailEngine
{
    public static class Extensions
    {
        public static async Task ForEachWithDelay<T>(this ICollection<T> items, Func<T, Task> action, double interval)
        {
            using (var timer = new System.Timers.Timer(interval))
            {
                var task = new Task(() => { });
                int remaining = items.Count;
                var queue = new ConcurrentQueue<T>(items);

                timer.Elapsed += async (sender, args) =>
                {
                    if (queue.TryDequeue(out T item))
                    {
                        try
                        {
                            await action(item);
                        }
                        finally
                        {
                            // Complete task.
                            remaining -= 1;

                            if (remaining == 0)
                            {
                                // No more items to process. Complete task.
                                task.Start();
                            }
                        }
                    }
                };

                timer.Start();

                await task;
            }
        }
    }
}