Syntax error through client.send() after successful authentication

I get a MailKit.Net.Smtp.SmtpCommandException: 'Syntax error, command unrecognized' crash on sending a message after a successful SMTP authentication.

Connected to ...
S: 220 ... ESMTP outbound-server; Fri, 21 Oct 2022 23:23:16 +0200
C: EHLO Der-Geraet
S: 250-... Hello Der-Geraet [/...] , Pleased to meet you
S: 250-DSN
S: 250-HELP
S: 250 AUTH LOGIN DIGEST-MD5 PLAIN CRAM-MD5
C: AUTH DIGEST-MD5
S: 334 ...
C: ********
S: 334 ...
C:
S: 235 Authentication successful
C: MAIL FROM:<[e-mail]>
S: 500 Syntax error, command unrecognized
C: RSET
S: 250 Requested mail action okay, completed
Unhandled exception. MailKit.Net.Smtp.SmtpCommandException: Syntax error, command unrecognized
   at MailKit.Net.Smtp.SmtpClient.OnSenderNotAccepted(MimeMessage message, MailboxAddress mailbox, SmtpResponse response)
   at MailKit.Net.Smtp.SmtpClient.ProcessMailFromResponse(MimeMessage message, MailboxAddress mailbox, SmtpResponse response)
   at MailKit.Net.Smtp.SmtpClient.MailFromAsync(FormatOptions options, MimeMessage message, MailboxAddress mailbox, SmtpExtension extensions, Int64 size, Boolean doAsync, CancellationToken cancellationToken)
   at MailKit.Net.Smtp.SmtpClient.SendAsync(FormatOptions options, MimeMessage message, MailboxAddress sender, IList`1 recipients, Boolean doAsync, CancellationToken cancellationToken, ITransferProgress progress)
   at MailKit.Net.Smtp.SmtpClient.SendAsync(FormatOptions options, MimeMessage message, MailboxAddress sender, IList`1 recipients, Boolean doAsync, CancellationToken cancellationToken, ITransferProgress progress)
   at MailKit.Net.Smtp.SmtpClient.Send(FormatOptions options, MimeMessage message, CancellationToken cancellationToken, ITransferProgress progress)
   at MailKit.MailTransport.Send(MimeMessage message, CancellationToken cancellationToken, ITransferProgress progress)
   at TestClient.Program.Main(String[] args) in ... .cs:line 36

I use the Sample Code for sending a message.

[jstedfast]

Yea your python code is not using DIGEST-MD5.

[jstedfast]

Was at dinner before using my phone, so had to wait until now to respond with some sample code that should work just like the python code:

using (var client = new SmtpClient ()) {
    client.Connect (hostName, 465, SecureSocketOptions.SslOnConnect);

    // Enforce LOGIN authentication:
    var login = new SaslMechanismLogin (username, password);
    client.Authenticate (login);

    client.Send (message);
    client.Disconnect (true);
}