Sending to Postal SMTP fails in MailKit 3.4.2

Describe the bug I’m trying to send emails to Postal (self-hosts SES alternative, more or less) via SMTP. This worked without a problem in MailKit 3.4.1 but stopped working in 3.4.2. On the server I can see the EHLO, but the connection is closed afterwards. The behaviour is the same with and without TLS enabled. I’d be happy to provide credentials for a test-organization in a private message/email.

Platform (please complete the following information):

• OS: Windows 11, Windows Server 2019
• .NET Runtime: CoreCLR
• .NET Framework: .NET 7.0
• MailKit Version: 3.4.2

Exception

Unhandled exception. MailKit.Net.Smtp.SmtpProtocolException: Unable to parse status code returned by the server.
   at MailKit.Net.Smtp.SmtpStream.ReadResponse(ByteArrayBuilder builder, Boolean& newLine, Boolean& more, Int32& code)
   at MailKit.Net.Smtp.SmtpStream.ReadResponseAsync(CancellationToken cancellationToken)
   at MailKit.Net.Smtp.SmtpStream.SendCommandAsync(String command, CancellationToken cancellationToken)
   at MailKit.Net.Smtp.SmtpClient.EhloAsync(CancellationToken cancellationToken)
   at MailKit.Net.Smtp.SmtpClient.ConnectAsync(String host, Int32 port, SecureSocketOptions options, CancellationToken cancellationToken)
   at Program.<Main>$(String[] args) in C:\Users\stipps\RiderProjects\MailTester\MailTester\Program.cs:line 15
   at Program.<Main>(String[] args)

To Reproduce Steps to reproduce the behavior:

1. Setup Postal instance (https://docs.postalserver.io/install/installation)
2. Create .NET console application
3. Add minimal code to send email over postal smtp
4. See error

Expected behavior MailKit connects to the server and sends the email.

Code Snippets If applicable, add code snippets to help explain your problem.

using MailKit;
using MailKit.Net.Smtp;
using MailKit.Security;
using MimeKit;

var msg = new MimeMessage();
msg.From.Add(InternetAddress.Parse("[e-mail]"));
msg.To.Add(InternetAddress.Parse("[e-mail]"));
msg.Subject = "Ihre Bestellung im Shop";

var bb = new BodyBuilder();
bb.TextBody = "Vielen Dank fuer Ihre Bestellung!";
msg.Body = bb.ToMessageBody();

var client = new SmtpClient();
await client.ConnectAsync("mailer.mydomain.com", 25, SecureSocketOptions.Auto);

await client.AuthenticateAsync("myorg/mydomain", "mypassword");
await client.SendAsync(msg);
await client.DisconnectAsync(true);

Protocol Logs

Connected to smtp://mailer.mydomain.com:25/?starttls=when-available
S: 220 mailer.haarhoff.eu ESMTP Postal/DPNZMRC: EHLO TICK

After this the exception is thrown.

Postal SMTP log:

postal-smtp-1  | [9] [2022-11-18T07:46:55.818] DEBUG -- : [YWMLLX]    Connection opened from ::ffff:176.199.38.70
postal-smtp-1  | [9] [2022-11-18T07:46:55.818] DEBUG -- : [YWMLLX]    Client identified as ::ffff:176.199.38.70
postal-smtp-1  | [9] [2022-11-18T07:46:55.839] DEBUG -- : [YWMLLX] <= EHLO TICK
postal-smtp-1  | [9] [2022-11-18T07:46:55.843] DEBUG -- : [YWMLLX] => 250-My capabilities are
postal-smtp-1  | [9] [2022-11-18T07:46:55.843] DEBUG -- : [YWMLLX] => 250-STARTTLS
postal-smtp-1  | [9] [2022-11-18T07:46:55.843] DEBUG -- : [YWMLLX] => 250 AUTH CRAM-MD5 PLAIN LOGIN
postal-smtp-1  | [9] [2022-11-18T07:46:55.856] DEBUG -- : [YWMLLX]    Connection closed

[jstedfast]

Awesome, thanks for testing my changes! I’ll try to make a new 3.4.3 release by early next week (after the holidays).

[haarhoff-frs]

Looks good, all my tests with our Postal server are now able to connect and send using that latest commit. Thank you!