What is SmtpServer?

SmtpServer is a simple, but highly functional SMTP server implementation. Written entirely in C# it takes full advantage of the .NET TPL to achieve maximum performance.

SmtpServer is available via NuGet

Whats New?

See here for whats new in Version 8.

What does it support?

SmtpServer currently supports the following ESMTP extensions:

• STARTTLS
• SIZE
• PIPELINING
• 8BITMIME
• AUTH PLAIN LOGIN

How can it be used?

At its most basic, it only takes a few lines of code for the server to be listening to incoming requests.

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

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

What hooks are provided?

There are three hooks that can be implemented; IMessageStore, IMailboxFilter, and IUserAuthenticator.

var options = new SmtpServerOptionsBuilder()
    .ServerName("localhost")
    .Port(25, 587)
    .Port(465, isSecure: true)
    .Certificate(CreateX509Certificate2())
    .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);

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();
    }
}