The implementation of the in-memory POP3 server.

Pop3Server.cs

using System;
using System.IO;
using System.Net;
using System.Text;
using LumiSoft.Net;
using LumiSoft.Net.POP3.Server;

namespace MockEmail
{
    public class Pop3Server : IDisposable
    {
        private readonly POP3_Server server = new POP3_Server();

        public Pop3Server(string message)
        {
            server.Bindings = new[] {new IPBindInfo("localhost", BindInfoProtocol.TCP, IPAddress.Loopback, 110)};

            server.SessionCreated += (sender, args) =>
                {
                    var session = args.Session;
                    session.Authenticate += (o, authenticate) => authenticate.IsAuthenticated = true;
                    session.GetMessagesInfo +=
                        (o, info) =>
                        info.Messages.Add(new POP3_ServerMessage(Guid.NewGuid().ToString(), message.Length));
                    session.GetMessageStream +=
                        (o, stream) => stream.MessageStream = new MemoryStream(Encoding.UTF8.GetBytes(message));
                };
        }

        public void Start()
        {
            server.Start();
        }

        public void Dispose()
        {
            server.Stop();
        }
    }
}