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

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);
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())
    .MessageStore(new SampleMessageStore())
    .MailboxFilter(new SampleMailboxFilter())
    .UserAuthenticator(new SampleUserAuthenticator())
    .Build();

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

public class SampleMessageStore : MessageStore
{
    public Task<SmtpResponse> SaveAsync(ISessionContext context, IMessageTransaction transaction, CancellationToken cancellationToken)
    {
        var textMessage = (ITextMessage)transaction.Message;
        
        var message = MimeKit.MimeMessage.Load(textMessage.Content);
        Console.WriteLine(message.TextBody);
    
        return Task.FromResult(SmtpResponse.Ok);
    }
}

public class SampleMailboxFilter : IMailboxFilter
{
    public Task<MailboxFilterResult> CanAcceptFromAsync(ISessionContext context, IMailbox @from, int size = 0)
    {
        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)
    {
        return Task.FromResult(MailboxFilterResult.Yes);
    }
}

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

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