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MailKit: The handshake failed due to an unexpected packet format

I got exception when I try to connect to my SMTP server using MailKit SmtpClient. BUT my mails have been sent successfully if I use System.Net.Mail.SmtpClient with the same parameters!
The exception message: An error occurred while attempting to establish an SSL or TLS connection.
The inner exception message: The handshake failed due to an unexpected packet format.
Questions
Why does MailKit.Net.Smtp.SmtpClient throw exception but System.Net.Mail.SmtpClient doesn't? What is the difference between them?
How to fix it?
Code
Initialize the parameters required for mail sending:
var host = "myhost.com";
var port = 2525;
var from = "from#mydomain.com";
var to = "to#mydomain.com";
var username = "from#mydomain.com";
var password = "myPassword";
var enableSsl = true;
Sending mail using System.Net.Mail.SmtpClient:
var client = new System.Net.Mail.SmtpClient
{
Host = host,
Port = port,
EnableSsl = enableSsl,
Credentials = new NetworkCredential(username, password)
};
client.Send(from, to, "subject", "body"); // success.
But when I try to connect to the host using MailKit with the same host and port, I got the exception:
var mailKitClient = new MailKit.Net.Smtp.SmtpClient();
mailKitClient.Connect(host, port, enableSsl); // it throws the exception.

The problem is that you are connecting to a plain-text port and expecting SSL.
In MailKit, the true/false useSsl parameter is used to decide whether or not to connect in SSL mode or plain-text mode.
In System.Net.Mail, they don't support connecting in SSL mode, they only support upgrading a plain-text connection to SSL mode using the STARTTLS command once the connection has been established.
To overcome this, MailKit has a different Connect() method that takes an enum value SecureSocketOptions.
What you want is SecureSocketOptions.StartTls:
var mailKitClient = new MailKit.Net.Smtp.SmtpClient();
mailKitClient.Connect(host, port, SecureSOcketOptions.StartTls);

Mailkit SMTP - StartTLS & TLS flags

I am trying to connect to iCloud via SmtpClient
The settings I am using are as follows:
Server name: smtp.mail.me.com
SSL Required: Yes
If you see an error message when using SSL, try using TLS or STARTTLS instead.
Port: 587
SMTP Authentication Required: Yes - with relevant username and password
If I use SSL I get "Handshake failed due to unexpected packet format"
If I don't use SSL visual studio debugger hangs on connect.
I think the problem is I am not telling the SmtpClient to use tls but I cant find documentation on how to do this.
The code is as follows:
using (var client = new SmtpClient()) {
client.Timeout = 1000 * 20;
//client.Capabilities.
client.AuthenticationMechanisms.Remove ("XOAUTH2");
client.Connect("SMTP.mail.me.com", 587, false); //dies here
//client.Connect(servername, port, useSsl);
//can I set tls or starttls here??
client.Authenticate(username, password);
client.Send(FormatOptions.Default, message);
}
Am I able to set TLS or StartTLS manually. One thing I did try is the following but it did not seem to work
client.Connect(new Uri("smtp://" + servername + ":" + port + "/?starttls=true"));
Thanks for any help with this.

The Connect() method that you are using only allows enabling/disabling SSL-wrapped connections which is not the same thing as StartTLS.
Due to the confusion, I've implemented a separate Connect() method that makes this more obvious what is going on:
using (var client = new SmtpClient()) {
// Note: don't set a timeout unless you REALLY know what you are doing.
//client.Timeout = 1000 * 20;
client.Connect ("smtp.mail.me.com", 587, SecureSocketOptions.StartTls);
client.Authenticate (username, password);
client.Send (message);
}
Try that.

You can set your options to "SecureSocketOptions.Auto"
something like this
await client.ConnectAsync(mailService.Host, mailService.Port, SecureSocketOptions.Auto);
MailKit will automatically decide to use SSL or TLS.

According to MailKit Doc
Connect(string host,int port,bool useSsl,CancellationToken cancellationToken = null)
The useSsl argument only controls whether or not the client makes an SSL-wrapped connection. In other words, even if the useSsl parameter is false, SSL/TLS may still be used if the mail server supports the STARTTLS extension.

This worked for me over the SecureSocketOptions.Auto option mentioned previously:
client.Connect(host, port, SecureSocketOptions.None);

C# SMTP virtual server doesn't send mail

C# SMTP virtual server doesn't send mail. Here's the code:
public static void SendEmail(string _FromEmail, string _ToEmail, string _Subject, string _EmailBody)
{
// setup email header .
SmtpMail.SmtpServer = "127.0.0.1";
MailMessage _MailMessage = new MailMessage();
_MailMessage.From = _FromEmail;
_MailMessage.To = _ToEmail;
_MailMessage.Subject = _Subject;
_MailMessage.Body = _EmailBody;
try
{
SmtpMail.Send(_MailMessage);
}
catch (Exception ex)
{
if (ex.InnerException != null)
{
String str = ex.InnerException.ToString();
}
}

It is an Interop exception because that .NET code is relying on Interop(erability) services, non-managed stuff. The message however is pretty clear, the server is unable to relay because it has been configured to reject relaying of emails.
Many years ago that was not a problem and you could virtually use any public SMTP server to send emails. With the advent of SPAM the whole game changed, now in most servers relaying is disabled and your mail send request is rejected.
What you will need here so that it does not reject your mail, is to authenticate your request (account/user and password on that server). It has to be a valid username & password combination known to that SMTP server. You do that by setting those (out of my head sorry) in the .Credentials property. See also the UseDefaultCredentials property, if you set Credentials you need to make sure UseDefaultCredentials is false.

If the error is this:
550 5.7.1 Unable to relay for ragaei.mahmoud#invensys.com
Then your SMTP server cannot send to that email address. Use a different SMTP server or configure it to relay out to the internet. The local one will probably only send to addresses on your own domain. If this is an SMTP server configuration question, you may have more luck asking on Superuser.

C# SMTP email sending code fails for Yahoo Mail but works fine for other servers, can anyone help?

I am using this code to send an SMTP email via the yahoo SMTP server, it is for a personal project I am writing.
using System.Net.Mail;
using System.Net;
SmtpClient theClient = new SmtpClient("smtp.mail.yahoo.com", 465);
theClient.UseDefaultCredentials = false;
theClient.Credentials = new NetworkCredential("username", "password");
theClient.EnableSsl = true;
MailMessage theMessage = new MailMessage("username#yahoo.com",
"to.someone#gmail.com");
theMessage.Subject = "Dave test from C# subject";
theMessage.Body = "Dave test from C# body";
theClient.Send(theMessage);
It's all pretty standard code for sending SMTP email, but... the server seems to throw an error. It forcibly terminates the connection. This does not happen if I use other SMTP servers like Gmail, Windows Live or various other ISP Smtp servers.
This is the exception and stack trace:
System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at ConsoleApplication1.Program.Main(String[] args) in E:\dev\ARCSoftware.FTPProcessor\ConsoleApplication1\Program.cs:line 28
I know the problem is not environmental though as I can send to the same server with these exact settings using Outlook Express. I am wondering if I need to send a certificate or something?
If you, or anyone you know where has any ideas about this I would greatly appreciate some help.

using System.Net.Mail;
using System.Net;
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private void btn_Send_Click(object sender, RoutedEventArgs e)
{
MailMessage oMail = new MailMessage(new MailAddress("username#yahoo.com"), new MailAddress("username#yahoo.com"));
SmtpClient oSmtp = new SmtpClient();
oSmtp.Host = "smtp.mail.yahoo.com";
oSmtp.Credentials = new NetworkCredential("username", "password");
oSmtp.EnableSsl = false;
oSmtp.Port = 587;
oSmtp.Send(oMail);
}
}

It's not supported through 465, but the following post details a workaround
How can I send emails through SSL SMTP with the .NET Framework?
UPDATE: This link details why it might work through Outlook Express, but not through the System.Net.Mail
http://blogs.msdn.com/b/webdav_101/archive/2008/06/02/system-net-mail-with-ssl-to-authenticate-against-port-465.aspx

Port 465 isn't supported by System.Net.Mail.SmtpClient.
http://msdn.microsoft.com/en-us/library/system.net.mail.smtpclient.enablessl.aspx
From the Remarks Section:
This connection method is sometimes called SMTP/SSL, SMTP over SSL, or SMTPS and by default uses port 465. This alternate connection method using SSL is not currently supported.
Edit:
You could try using port 587 instead (if Yahoo supports it).

I had the same problem until I set the port to 587 and disabled SSL.

I think you should revert to using System.Web.Mail which lets you control fields that are not accessible through the newer System.Net.
Try to play with those. For instance you could try this:
(use is documented here, fields are documented here)
MailMessage msg = new MailMessage();
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserver", "smtp.mail.yahoo.com");
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserverport", "465");
// try "2", I have not tested for yahoo mail
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusing", "2");
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpusessl", "1");
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1"); // 0= anonymous - 1=basic - 2=NTLM
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", "yahoousername");
msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", "yahoopwd");

Troubleshooting "The server committed a protocol violation" when sending mail with SmtpClient

I want to send a mail message with the SmtpClient class.
Here's the code I use:
SmtpClient smtpClient = new SmtpClient("Host",25);
NetworkCredential basicCredential =
new NetworkCredential("UserName", "Password");
MailMessage message = new MailMessage();
MailAddress fromAddress = new MailAddress("me#domain.com");
smtpClient.UseDefaultCredentials = false;
smtpClient.Credentials = basicCredential;
message.From = fromAddress;
message.Subject = "test send";
message.IsBodyHtml = true;
message.Body = "<h1>hello</h1>";
message.To.Add("mail#domain.com");
smtpClient.Send(message);
But it always throws an exception:
The server committed a protocol violation The server response was: UGFzc3dvcmQ6
I can't find the reason for that. Please, if anyone has faced something like this, tell me what to do.

I had the same problem, for my case it was for setting user#domain instead of user, I mean
Old code
new NetworkCredential("UserName#domain.com", "Password");
New code
new NetworkCredential("UserName", "Password");

This looks to me like SmtpClient authentication is somehow getting out of step.
Some authentication mechanisms are "Client: request auth with username and password, Server: success/fail" others are "Client: request auth with username, Server: request password, Client: reply with password, Server: success/fail".
It looks like SmtpClient is expecting the former, while your server is expecting the latter.
As dave wenta suggested, a log of a session would tell you what auth mechanism SmtpClient is trying to use, but it will also say what auth mechanisms the server supports.
What normally happens is that the server offers a number of authetication options, and the client choses which one it is going to use. The behaviour from there should be determined by the protocol chosen. I would hope that the SmtpClient class took care of that for you though, but I'm afraid I've never used that particular class.
Also remember - If you are going to post a log here, change to a throwaway password before you log the session, as a base64 encoded plain text password can be trivially changed back to human readable plain text password.

UGFzc3dvcmQ6 is "Password:" base 64 encoded (without quotes), meaning the password is probably wrong or not send encoded. Try base 64 encoding the password:
string password = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes("Password));

Enable logging for System.Net.Mail. Then view the log file. This will show you exactly what is happening during the SMTP layer.
Here is a link with more info:
http://systemnetmail.com/faq/4.10.aspx

thousand of hit on google for UGFzc3dvcmQ6
it seem the server expect encrypted(with base64) username/password

Had the same problem. Solved it in the following steps:
1) find out, what the server offers for SMTP Authentication by connecting to the SMTP Server using Telnet or putty or any other terminal:
telnet xxx.yyy.zzz.aaa 587
(xxx.yyy.zzz.aaa = IP address of the SMTP Server, 587 = port number)
< Server answers with "220 protocol + version + time"
ehlo testing
< Server displays list of capabilities e.g.
250-AUTH NTLM CRAM-MD5 LOGIN
The SMTP client tries to take the most secure protocol first. In my case:
1. System.Net.Mail.SmtpNegotiateAuthenticationModule
2. System.Net.Mail.SmtpNtlmAuthenticationModule
3. System.Net.Mail.SmtpDigestAuthenticationModule
4. System.Net.Mail.SmtpLoginAuthenticationModule
It looks as if the SMTP client tries NTLM while the server tries to run LOGIN.
With a hack (cf. https://blogs.msdn.microsoft.com/knom/2008/04/16/hacking-system-net-mail-smtpclient/), all protocols can be turned of except for the one the server assumes (LOGIN in this case):
FieldInfo transport = smtpClient.GetType().GetField("transport", BindingFlags.NonPublic | BindingFlags.Instance);
FieldInfo authModules = transport.GetValue(smtpClient).GetType().GetField("authenticationModules",BindingFlags.NonPublic | BindingFlags.Instance);
Array modulesArray = authModules.GetValue(transport.GetValue(smtpClient)) as Array;
foreach (var module in modulesArray)
{
Console.WriteLine(module.ToString());
}
// System.Net.Mail.SmtpNegotiateAuthenticationModule
// System.Net.Mail.SmtpNtlmAuthenticationModule
// System.Net.Mail.SmtpDigestAuthenticationModule
// System.Net.Mail.SmtpLoginAuthenticationModule
// overwrite the protocols that you don't want
modulesArray.SetValue(modulesArray.GetValue(3), 0);
modulesArray.SetValue(modulesArray.GetValue(3), 1);
modulesArray.SetValue(modulesArray.GetValue(3), 2);

This can also happen when you simply don't provide the password. In my case I was using credentials from a web.config smtp block and on my deployment server (using octopus deploy) had forgotten to populate the password attribute.

For anyone else coming across this on google.. I fixed this by supplying my username in the "username" format instead of "DOMAIN\username"
$Credential = Get-Credential
Send-MailMessage -SmtpServer exchange.contoso.com -Credential $Credential -From 'user#contoso.com' -To 'recipient#domain.com' -Subject 'Test email' -Port 587 -UseSsl