var options = new SmtpServerOptionsBuilder()
    .ServerName("localhost")
    .Port(25, 587)
    .Build();

var smtpServer = new SmtpServer.SmtpServer(options, ServiceProvider.Default);
await smtpServer.StartAsync(CancellationToken.None);

var options = new SmtpServerOptionsBuilder()
    .ServerName("localhost")
    .Endpoint(builder =>
        builder
            .Port(9025, true)
            .AllowUnsecureAuthentication(false)
            .Certificate(CreateCertificate()))
    .Build();

var serviceProvider = new ServiceProvider();
serviceProvider.Add(new SampleMessageStore());
serviceProvider.Add(new SampleMailboxFilter());
serviceProvider.Add(new SampleUserAuthenticator());

var smtpServer = new SmtpServer.SmtpServer(options, serviceProvider);
await smtpServer.StartAsync(CancellationToken.None);

// to create an X509Certificate for testing you need to run MAKECERT.EXE and then PVK2PFX.EXE
// http://www.digitallycreated.net/Blog/38/using-makecert-to-create-certificates-for-development
static X509Certificate2 CreateCertificate()
{
    var certificate = File.ReadAllBytes(@"Certificate.pfx");

    return new X509Certificate2(certificate, "P@ssw0rd");
}

public class SampleMessageStore : MessageStore
{
    public override async Task<SmtpResponse> SaveAsync(ISessionContext context, IMessageTransaction transaction, ReadOnlySequence<byte> buffer, CancellationToken cancellationToken)
        {
            await using var stream = new MemoryStream();

            var position = buffer.GetPosition(0);
            while (buffer.TryGet(ref position, out var memory))
            {
                await stream.WriteAsync(memory, cancellationToken);
            }

            stream.Position = 0;

            var message = await MimeKit.MimeMessage.LoadAsync(stream, cancellationToken);
            Console.WriteLine(message.TextBody);

            return SmtpResponse.Ok;
        }
}

public class SampleMailboxFilter : IMailboxFilter, IMailboxFilterFactory
{
    public Task<MailboxFilterResult> CanAcceptFromAsync(ISessionContext context, IMailbox @from, int size, CancellationToken cancellationToken)
    {
        if (String.Equals(@from.Host, "test.com"))
        {
            return Task.FromResult(MailboxFilterResult.Yes);
        }

        return Task.FromResult(MailboxFilterResult.NoPermanently);
    }

    public Task<MailboxFilterResult> CanDeliverToAsync(ISessionContext context, IMailbox to, IMailbox @from, CancellationToken token)
    {
        return Task.FromResult(MailboxFilterResult.Yes);
    }

    public IMailboxFilter CreateInstance(ISessionContext context)
    {
	return new SampleMailboxFilter();
    }
}

public class SampleUserAuthenticator : IUserAuthenticator, IUserAuthenticatorFactory
{
    public Task<bool> AuthenticateAsync(ISessionContext context, string user, string password, CancellationToken token)
    {
        Console.WriteLine("User={0} Password={1}", user, password);

        return Task.FromResult(user.Length > 4);
    }

    public IUserAuthenticator CreateInstance(ISessionContext context)
    {
	return new SampleUserAuthenticator();
    }
}