Create C# SMTP Server

It took a while to find a library which provides a SMTP server. There are a lot of SMTP clients out there, but the number of SMTP servers seems very limited.

Finally, I found a library SmtpServer with an SMTP server and several hooks to extend the functionality.

Open port 25

The documentation is straightforward and well-documented.

	var options = new SmtpServerOptionsBuilder()
	    .Port(25, 587)
	    .Port(465, isSecure: true)
	// this is used to intercept the processing and inject hooks
	var serviceProvider = new ServiceProvider();
	serviceProvider.Add(new SampleMessageStore());
	serviceProvider.Add(new SampleMailboxFilter());
	serviceProvider.Add(new SampleUserAuthenticator());
    // lets start the server and wait
	var smtpServer = new SmtpServer.SmtpServer(options, serviceProvider);
	await smtpServer.StartAsync(CancellationToken.None);

Wait for Mail

This example reads the MimePart of the attachment and shows the attachments filename.

	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("Mail received: " + message.Subject);
	        foreach (var attachment in message.Attachments)
	            Console.WriteLine("...Attachment found: " + (attachment as MimePart)?.FileName);
	        return SmtpResponse.Ok;

Send Mail

For sending am email, a PowerShell commandlet can be used.

Send-MailMessage -To -Subject "Hello User" -Body "This is the message" -SmtpServer "localhost" -From "" -Attachments .\Documents\IMG-20220425-WA0058.jpg




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