Unable to read data from the transport connection: net_io_connectionclosed - simple solution when encountering the error

Question

How to solve the problem - unable to read data from the transport connection: net_io_connectionclosed? I have the sample detail:

SmtpClient client = new SmtpClient();
 client.Host = "hostname";
 client.Port = 465;
 client.DeliveryMethod = SmtpDeliveryMethod.Network;
 client.UseDefaultCredentials = false;
 client.EnableSsl = true;
 client.Credentials = new NetworkCredential("User", "Pass);
 client.Send("from@hostname", "to@hostname", "Subject", "Body");

While I was running it, I found the warning message:

Unable to read data from the transport connection: net_io_connectionclosed

That is my question in my midterm exam, and it is urgent. I searched the solutions on some websites, but I didn't get it. I may miss any line or other changes. I appreciate your assistance!

dttutoria · Accepted Answer

The cause: The problem is using port 465 which is clearly deprecated.

Solution: Instead of 465, you can use port 587. Lessons that we may apply are as follows: - STARTTLS is the only encryption protocol that the.NET SmtpClient supports. A STARTTLS response to EHLO is required if the EnableSsl option is set; otherwise, the server will raise an exception. - Plaintext and encryption can both be used on the same port by using STARTTLS to support services. An encrypted connection would be established after switching from plaintext with the STARTTLS command. For SMTP encryption, STARTTLS has become the criterion.

Raphaël Denis · Answer

This post is for anyone who came across it looking for a solution.

The username is not the username that you created when you created the sendgrid object. Find your username

Click on the sendgrid object you wish to manage in azure. You will be redirected back to SendGrid.
After you confirm your email, copy the username that appears. It's an automatic generated username.
SendGrid username to your SMTP settings in web.config.

We hope this helps!

Isaac O'Kelly · Answer

I tried everything, but the error still occurs with Office 365 account. This code works fine with Google account.

Are there any other ideas?

Here's the code I am using

int port = 587;
string host = "smtp.office365.com";
string username = "smtp.out@mail.com";
string password = "password";
string mailFrom = "noreply@mail.com";
string mailTo = "to@mail.com";
string mailTitle = "Testtitle";
string mailMessage = "Testmessage";

using (SmtpClient client = new SmtpClient())
{
 MailAddress from = new MailAddress(mailFrom);
 MailMessage message = new MailMessage
 {
 From = from
 };
 message.To.Add(mailTo);
 message.Subject = mailTitle;
 message.Body = mailMessage;
 message.IsBodyHtml = true;
 client.DeliveryMethod = SmtpDeliveryMethod.Network;
 client.UseDefaultCredentials = false;
 client.Host = host;
 client.Port = port;
 client.EnableSsl = true;
 client.Credentials = new NetworkCredential
 {
 UserName = username,
 Password = password
 }; 
 client.Send(message);
}

UPDATE AND HOW IT WAS SOLVED:

Problem solved by switching from System.Net.Mail Smtp Client on to Mailkit Microsoft has stopped recommending the System.Net.Mail Smtp client because of security concerns. Instead, you should use MailKit. Mailkit provided clearer error messages, which helped me understand the root cause (a license issue). Mailkit can be downloaded as a Nuget package.

Read documentation about Smtp Client for more information: https://docs.microsoft.com/es-es/dotnet/api/system.net.mail.smtpclient?redirectedfrom=MSDN&view=netframework-4.7.2

Here's how I implemented SmtpClient using MailKit

 int port = 587;
 string host = "smtp.office365.com";
 string username = "smtp.out@mail.com";
 string password = "password";
 string mailFrom = "noreply@mail.com";
 string mailTo = "mailto@mail.com";
 string mailTitle = "Testtitle";
 string mailMessage = "Testmessage";

 var message = new MimeMessage();
 message.From.Add(new MailboxAddress(mailFrom));
 message.To.Add(new MailboxAddress(mailTo));
 message.Subject = mailTitle;
 message.Body = new TextPart("plain") { Text = mailMessage };

 using (var client = new SmtpClient())
 {
 client.Connect(host , port, SecureSocketOptions.StartTls);
 client.Authenticate(username, password);

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

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Unable to read data from the transport connection: net_io_connectionclosed – simple solution when encountering the error