CSharp-SMTP-Server 1.1.1

CSharp-SMTP-Server

Simple (receive only) SMTP server library for C#.

This server is only returning all received emails to interface provided by the software running this library.

Supported features

• TLS and STARTTLS
• AUTH LOGIN and AUTH PLAIN

Compatible with

• RFC 822 (STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES)
• RFC 1869 (SMTP Service Extensions)
• RFC 2554 (SMTP Service Extension for Authentication)
• RFC 3463 (Enhanced Mail System Status Codes)
• RFC 4616 (The PLAIN Simple Authentication and Security Layer (SASL) Mechanism)
• RFC 4954 (SMTP Service Extension for Authentication)
• RFC 5321 (SMTP Protocol)
• RFC 7208 (Sender Policy Framework)

Basic usage

var server = new SMTPServer(new[]
{
	new ListeningParameters(IPAddress.Any, new ushort[]{25, 587}, new ushort[]{465}),
	new ListeningParameters(IPAddress.IPv6Any, new ushort[]{25, 587}, new ushort[]{465})
}, new ServerOptions(){ServerName = "Test SMTP Server", RequireEncryptionForAuth = false}, new DeliveryInterface(), new LoggerInterface());
//with TLS:
//}, new ServerOptions() { ServerName = "Test SMTP Server", RequireEncryptionForAuth = true}, new DeliveryInterface(), new LoggerInterface(), new X509Certificate2("PathToCertWithKey.pfx"));
		
server.SetAuthLogin(new AuthenticationInterface());
server.SetFilter(new FilterInterface());
server.Start();

class LoggerInterface : ILogger
{
	public void LogError(string text) => Console.WriteLine("[LOG] " + text);
}

class DeliveryInterface : IMailDelivery
{
	//Let's just print all emails
	public Task EmailReceived(MailTransaction transaction)
	{
		Console.WriteLine(
			$"\n\n--- EMAIL TRANSACTION ---\nSource IP: {transaction.RemoteEndPoint}\nAuthenticated: {transaction.AuthenticatedUser ?? "(not authenticated)"}\nFrom: {transaction.From}\nTo (Commands): {transaction.DeliverTo.Aggregate((current, item) => current + ", " + item)}\nTo (Headers): {transaction.To.Aggregate((current, item) => current + ", " + item)}\nCc: {transaction.Cc.Aggregate((current, item) => current + ", " + item)}\nBcc: {transaction.Bcc.Aggregate((current, item) => current + ", " + item)}\nBody: {transaction.Body}\n--- END OF TRANSACTION ---\n\n");
		return Task.CompletedTask;
	}

	//We only own "@smtp.demo" and we don't want any emails to other domains
	public Task<UserExistsCodes> DoesUserExist(string emailAddress) => Task.FromResult(emailAddress.EndsWith("@smtp.demo", StringComparison.OrdinalIgnoreCase)
		? UserExistsCodes.DestinationAddressValid
		: UserExistsCodes.BadDestinationSystemAddress);
}

class AuthenticationInterface : IAuthLogin
{
	//123 is password for all users (NOT SECURE, ONLY FOR DEMO PURPOSES!)

	public Task<bool> AuthPlain(string authorizationIdentity, string authenticationIdentity, string password,
		EndPoint remoteEndPoint,
		bool secureConnection) => Task.FromResult(password == "123");

	public Task<bool> AuthLogin(string login, string password, EndPoint remoteEndPoint, bool secureConnection) =>
		Task.FromResult(password == "123");
}

class FilterInterface : IMailFilter
{
	//Allow all connections
	public Task<SmtpResult> IsConnectionAllowed(EndPoint ep) => Task.FromResult(new SmtpResult(SmtpResultType.Success));

	//Let's block .invalid TLD. You can do here eg. SPF validation
	public Task<SmtpResult> IsAllowedSender(string source, EndPoint ep) => Task.FromResult(source.TrimEnd().EndsWith(".invalid")
		? new SmtpResult(SmtpResultType.PermanentFail)
		: new SmtpResult(SmtpResultType.Success));
		
	//Let's reject Softfail as well
	public Task<SmtpResult> IsAllowedSenderSpfVerified(string source, EndPoint? ep, SpfResult spfResult) => Task.FromResult(spfResult == SpfResult.Softfail
		? new SmtpResult(SmtpResultType.PermanentFail)
		: new
			SmtpResult(SmtpResultType.Success));

	//Let's block all emails to root at any domain
	public Task<SmtpResult> CanDeliver(string source, string destination, bool authenticated, string username,
		EndPoint ep) => Task.FromResult(destination.TrimStart().StartsWith("root@", StringComparison.OrdinalIgnoreCase)
		? new SmtpResult(SmtpResultType.PermanentFail)
		: new SmtpResult(SmtpResultType.Success));

	//Let's blacklist word "spam"
	public Task<SmtpResult> CanProcessTransaction(MailTransaction transaction) => Task.FromResult(transaction.Body != null && transaction.Body.ToLower().Contains("spam", StringComparison.OrdinalIgnoreCase)
		? new SmtpResult(SmtpResultType.PermanentFail)
		: new SmtpResult(SmtpResultType.Success));
}

Generating PFX from PEM keys

You can generate PFX from PEM certificate and PEM private key using openssl:

openssl pkcs12 -export -in public.pem -inkey private.pem -out CertWithKey.pfx