Mailkit: Unable to send an email useSsl =false at local smtp server

[alirazaedev]

I m try to send an email using my smpt live server port 465 useSsl=true with below code it run correctly and email sent but when i try to send an email using my local smpt server port 25 useSsl=false.

Environment
language C#
window 10

Below is the code i m trying it throw an exception

MailKit.Net.Smtp.SmtpClient client = null;
using (client = new MailKit.Net.Smtp.SmtpClient())
                {

                    client.ServerCertificateValidationCallback = (s, c, h, e) => true;
                    client.Connect(IpAddress, 25, false);                  
                    client.Send("here is object of  MimeMessage");
                    client.Disconnect(true);
                    client.Dispose();                  
                } 

Exception at below
message: The SMTP server has unexpectedly disconnected.

StackTrace:   at MailKit.Net.Smtp.SmtpStream.<ReadAheadAsync>d__47.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MailKit.Net.Smtp.SmtpStream.<ReadResponseAsync>d__54.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MailKit.Net.Smtp.SmtpStream.ReadResponse(CancellationToken cancellationToken)
   at MailKit.Net.Smtp.SmtpClient.<DataAsync>d__97.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at MailKit.Net.Smtp.SmtpClient.<SendAsync>d__99.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MailKit.Net.Smtp.SmtpClient.Send(FormatOptions options, MimeMessage message, CancellationToken cancellationToken, ITransferProgress progress)
   at MailKit.MailTransport.Send(MimeMessage message, CancellationToken cancellationToken, ITransferProgress progress)

Please reply what is going wrong
i want to fix that

[alirazaedev]

Below are the version i m using
BouncyCastle.1.8.1
MailKit.2.0.3
MimeKit.2.0.3

[alirazaedev]

mail server is correctly setuped because before using mimekit I m using below code System.Net.Mail it runs correctly but it not sending email when set client.EnableSsl=true that is problem with below code

using (SmtpClient client = new SmtpClient(smtpSection.Network.Host))
{
client.EnableSsl = smtpSection.Network.EnableSsl;
client.Host = smtpSection.Network.Host;
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential(smtpSection.Network.UserName,
smtpSection.Network.Password);
client.Port = smtpSection.Network.Port;

             }

[alirazaedev]

whith local smpt server what is the code to send an email using mimekit
local smpt server use port=25 and enablesll=false

[jstedfast]

Based on the stack trace, it looks like the SmtpClient gets all the way to the DATA command before getting disconnected.

This means that it was able to successfully establish a connection with your local SMTP server, authenticate, and specify which recipients to send the message to, but the connection was lost for some reason during the actual upload of the message to the server (or, more likely based on the stack trace, it completely uploaded the message but the connection dropped as the client attempted to read the response from the server).

Typically when something like this happens, either:

1. There was a network outage for the server (e.g. a hickup with its router)
2. There was a network outage for the client (e.g. a hickup with your router)
3. The server dropped your connection for some other reason (reboot? out of resources? server bug? protocol violation in the client?)

In general, 1 and 2 aren't the problem if you are connecting to a local server, especially if it continues to happen, but I listed them because it's important not to completely dismiss them as possibilities to consider if other avenues don't pan out.

That leaves option 3 as the likely culprit. My first instinct is to ask how large the message is - is it a simple text-only message or does it have large attachment(s) that might exceed the server's message size limit (often 25MB, but sometimes less, sometimes more)?

If that doesn't seem to be a likely cause, my next suggestion is to send me a protocol log so that I can see if I can figure out what might have gone wrong (or at least verify that it's not a protocol violation on the client's side).

If you need to send me a protocol log, feel free to scrub the AUTH command data so that your credentials are safe (even if you email them to me, I won't use them or decode them, but no sense risking exposure of them unnecessarily and I highly doubt they will be necessary to diagnose the issue).

[jstedfast]

Oh, and since you know you'll be sending me a protocol log which will contain the message data, feel free to craft it such that it does not contain anything sensitive either.

[alirazaedev]

please find the attached log
smtp.log

[jstedfast]

Doesn't seem like the message would qualify as being too large for any SMTP server to accept, so I think we can safely rule out that potential explanation.

I noticed that the server supports authentication and that you didn't authenticate. Is it possible that the server is dropping your connection at the end of the DATA content being submitted because you didn't authenticate?

Truthfully, if that is the case, I would call shenanigans on the SMTP server for doing that since it should have errored out much sooner if it wants you to authenticate first, but SMTP servers do weird things sometimes.

I don't think there's a bug in MailKit's SmtpClient here, it seems to have gotten disconnected before MailKit could read a response from the server.

[alirazaedev]

1) I have an smtp server where ssl certificate installed that require authentication and for that my code is
using (var client = new MailKit.Net.Smtp.SmtpClient())
{
client.ServerCertificateValidationCallback = (s, c, h, e) => true;
client.Connect(host, port, ssl);
// Note: only needed if the SMTP server requires authentication
client.Authenticate(u, p);
client.Send(mailMessage);
client.Disconnect(true);
client.Dispose();
}

Parameter i m passing to client.Connect(host, port, ssl); is port=465 with ssl=true and port=25 with ssl=false in both cases it run correctly.

2) Problem I have an smtp server where ssl certificate not installed and that require no authentication, relay based user configured on that server code for that is below
using (var client = new MailKit.Net.Smtp.SmtpClient())
{
client.Connect(host, port, ssl);
client.Send(mailMessage);
client.Disconnect(true);
client.Dispose();
}
Parameter i m passing to client.Connect(host, port, ssl); is port=25 with ssl=false in this cases it throw an exception that i was mentioned in my first comment at top

3) Solution but not with mailkit I have an smtp server where ssl certificate not installed and that require no authentication, relay based user configured on that server code for that is below
using (System.Net.Mail.SmtpClient client = new System.Net.MailSmtpClient(host)
{
client.EnableSsl = false;
client.Port = 25
client.Send(mailMessage);
}
it run successfully it mean there is no problem with smtp server.

why i musing MailKit? Answer is below
using System.Net.Mail.SmtpClient class when i m sending email from server where ssl certificate installed that not working but using MailKit my problem resolve but mailkit not working with server where ssl certificate not installed and no authentication required.

[alirazaedev]

but i want to solution with Mailkit

[jstedfast]

Try this:

using (var client = new MailKit.Net.Smtp.SmtpClient())
{
    client.Connect(host, port, ssl);
    client.Capabilities &= ~SmtpCapabilities.Pipelining;
    client.Send(mailMessage);
    client.Disconnect(true);
}

[alirazaedev]

Thanks
try above code the issue resolved please can you explain client.Capabilities and can i go with this in production environment where different servers may support different type of capabilities.
how can i check a server support how many capabilities and and after connect what is the default values of capabilities.

[jstedfast]

The Capabilities property just specifies which SMTP extensions that the server claims to support. In your case, the server claims to support the PIPELINING extension but seems to break when the client actually uses it.

What my code above does is it removes the Pipelining flag from the property so that the SmtpClient doesn't try to use it.

You can disable any feature that the server supports using the above technique if you think it may be causing interoperability problems.

I suggested removing PIPELINING since I know some SMTP servers break when that extension gets used.

That's the only extension I know of that causes problems sometimes, so I don't think you have to worry about any others.

You will always be safe disabling PIPELINING (even if the server doesn't support it - it'll just end up being a no-op), so feel free to do that.

[wonea]

I'm using Papercut and cannot send use SMTP with MailKit, but sending a message from PowerShell (using Net.Mail.SmtpClient) does work.

MailKit error
mailkit syntax error, 3 retry(ies) remaining

using (var client = new MailKit.Net.Smtp.SmtpClient())
{
    client.Capabilities &= ~SmtpCapabilities.Pipelining;
    client.Connect("localhost", 25, false);
    client.Send(message);
    client.Disconnect(true);
}

If with MailKit I change the localhost to smtp://localhost then I no longer get a syntax error, but server is not found. Is there broken regex somewhere?

[jstedfast]

I don't know what that syntax error is, but it's not from MailKit. MailKit has 0 regex usage and does not do anything with the hostname except pass it to Socket.Connect().

You can see the socket connect logic here: https://github.com/jstedfast/MailKit/blob/master/MailKit/Net/SocketUtils.cs#L55

The host string is the exact string you passed into the client.Connect() method.

[jstedfast]

BTW, client.Capabilities &= ~SmtpCapabilities.Pipelining; is pointless to do before connecting / authenticating.

[wonea]

My code is running on netcoreapp2.2
Looking at the #If directives, surely netcoreapp2.2 will use netstandard2.0

[jstedfast]

Correct, that's what I would expect, too.

[vmakwana]

Similarly, i set the useSsl to false but the client.connect still tries to initiate SSl/TLS and fails on following error:

An error occurred while attempting to establish an SSL or TLS connection.

The SSL certificate presented by the server is not trusted by the system for one or more of the following reasons:

1. The server is using a self-signed certificate which cannot be verified.
2. The local system is missing a Root or Intermediate certificate needed to verify the server's certificate.
3. The certificate presented by the server is expired or invalid.

[jstedfast]

The useSsl argument only tells MailKit whether the port that you are connecting to is an SSL-wrapped port, e.g. port 465 for SMTP, port 993 for IMAP, or port 995 for POP3.

If you do not want MailKit to use the STARTTLS command when connecting to a plain-text port, then you need to use:

client.Connect (host, port, SecureSocketOptions.None);