SmtpException: Unable to read data from the transport connection: net_io_connectionclosed

Question

I am using the SmtpClient library to send emails using the following:

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");

The code works fine in my test environment, but when I use production SMTP servers, the code fails with an SmtpException "Failure sending mail." with an inner IOException "Unable to read data from the transport connection: net_io_connectionclosed".

I've confirmed that firewalls are not an issue. The port opens just fine between the client and the server. I'm not sure what else could throw this error.

Answer 1

EDIT: Super Redux Version

Try port 587 instead of 465. Port 465 is technically deprecated.

---

After a bunch of packet sniffing I figured it out. First, here's the short answer:

The .NET SmtpClient only supports encryption via STARTTLS. If the EnableSsl flag is set, the server must respond to EHLO with a STARTTLS, otherwise it will throw an exception. See the MSDN documentation for more details.

Second, a quick SMTP history lesson for those who stumble upon this problem in the future:

Back in the day, when services wanted to also offer encryption they were assigned a different port number, and on that port number they immediately initiated an SSL connection. As time went on they realized it was silly to waste two port numbers for one service and they devised a way for services to allow plaintext and encryption on the same port using STARTTLS. Communication would start using plaintext, then use the STARTTLS command to upgrade to an encrypted connection. STARTTLS became the standard for SMTP encryption. Unfortunately, as it always happens when a new standard is implemented, there is a hodgepodge of compatibility with all the clients and servers out there.

In my case, my user was trying to connect the software to a server that was forcing an immediate SSL connection, which is the legacy method that is not supported by Microsoft in .NET.

Answer 2

For anyone who stumbles across this post looking for a solution and you've set up SMTP sendgrid via Azure.

The username is not the username you set up when you've created the sendgrid object in azure. To find your username;

• Click on your sendgrid object in azure and click manage. You will be redirected to the SendGrid site.
• Confirm your email and then copy down the username displayed there.. it's an automatically generated username.
• Add the username from SendGrid into your SMTP settings in the web.config file.

Hope this helps!

Answer 3

Change port from 465 to 587 and it will work.

Answer 4

I've tried all the answers above but still get this error with Office 365 account. The code seems to work fine with Google account and smtp.gmail.com when allowing less secure apps.

Any other suggestions that I could try?

Here is the code that I'm using

int port = 587;
string host = "smtp.office365.com";
string username = "[e-mail]";
string password = "password";
string mailFrom = "[e-mail]";
string mailTo = "[e-mail]";
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 I SOLVED IT:

Solved problem by changing Smtp Client to Mailkit. The System.Net.Mail Smtp Client is now not recommended to use by Microsoft because of security issues and you should instead be using MailKit. Using Mailkit gave me clearer error messages that I could understand finding the root cause of the problem (license issue). You can get Mailkit by downloading it 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 is how I implemented SmtpClient with MailKit

        int port = 587;
        string host = "smtp.office365.com";
        string username = "[e-mail]";
        string password = "password";
        string mailFrom = "[e-mail]";
        string mailTo = "[e-mail]";
        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);
        }

Answer 5

You may also have to change the "less secure apps" setting on your Gmail account. EnableSsl, use port 587 and enable "less secure apps". If you google the less secure apps part there are google help pages that will link you right to the page for your account. That was my problem but everything is working now thanks to all the answers above.

Answer 6

Does your SMTP library supports encrypted connection ? The mail server might be expecting secure TLS connection and hence closing the connection in absence of a TLS handshake

Answer 7

If you are using an SMTP server on the same box and your SMTP is bound to an IP address instead of "Any Assigned" it may fail because it is trying to use an IP address (like 127.0.0.1) that SMTP is not currently working on.

Answer 8

removing

client.UseDefaultCredentials = false; 

seemed to solve it for me.

Answer 9

To elevate what jocull mentioned in a comment, I was doing everything mention in this thread and striking out... because mine was in a loop to be run over and over; after the first time through the loop, it would sometimes fail. Always worked the first time through the loop.

To be clear: the loop includes the creation of SmtpClient, and then doing .Send with the right data. The SmtpClient was created inside a try/catch block, to catch errors and to be sure the object got destroyed before the bottom of the loop.

In my case, the solution was to make sure that SmtpClient was disposed after each time in the loop (either via using() statement or by doing a manual dispose). Even if the SmtpClient object is being implicitly destroyed in the loop, .NET appears to be leaving stuff lying around to conflict with the next attempt.

Answer 10

Change your port number to 587 from 465

Answer 11

In my case, the customer forgot to add new IP address in their SMTP settings. Open IIS 6.0 in the server which sets up the smtp, right click Smtp virtual server, choose Properties, Access tab, click Connections, add IP address of the new server. Then click Relay, also add IP address of the new server. This solved my issue.

Answer 12

I ran into this when using smtp.office365.com, using port 587 with SSL. I was able to log in to the account using portal.office.com and I could confirm the account had a license. But when I fired the code to send emails, I kept getting the net_io_connectionclosed error.

Took me some time to figure it out, but the Exchange admin found the culprit. We're using O365 but the Exchange server was in a hybrid environment. Although the account we were trying to use was synced to Azure AD and had a valid O365 license, for some reason the mailbox was still residing on the hybrid Exchange server - not Exchange online. After the exchange admin used the "Move-Mailbox" command to move the mailbox from the hybrid exchange server to O365 we could use the code to send emails using o365.

Answer 13

Try this : Here is the code which i'm using to send emails to multiple user.

 public string gmail_send()
    {
        using (MailMessage mailMessage =
        new MailMessage(new MailAddress(toemail),
    new MailAddress(toemail)))
        {
            mailMessage.Body = body;
            mailMessage.Subject = subject;
            try
            {
                SmtpClient SmtpServer = new SmtpClient();
                SmtpServer.Credentials =
                    new System.Net.NetworkCredential(email, password);
                SmtpServer.Port = 587;
                SmtpServer.Host = "smtp.gmail.com";
                SmtpServer.EnableSsl = true;
                mail = new MailMessage();
                String[] addr = toemail.Split(','); // toemail is a string which contains many email address separated by comma
                mail.From = new MailAddress(email);
                Byte i;
                for (i = 0; i < addr.Length; i++)
                    mail.To.Add(addr[i]);
                mail.Subject = subject;
                mail.Body = body;
                mail.IsBodyHtml = true;
                mail.DeliveryNotificationOptions =
                    DeliveryNotificationOptions.OnFailure;
                //   mail.ReplyTo = new MailAddress(toemail);
                mail.ReplyToList.Add(toemail);
                SmtpServer.Send(mail);
                return "Mail Sent";
            }
            catch (Exception ex)
            {
                string exp = ex.ToString();
                return "Mail Not Sent ... and ther error is " + exp;
            }
        }
    }

Answer 14

In case if all above solutions don't work for you then try to update following file to your server (by publish i mean, and a build before that would be helpful).

bin-> projectname.dll 

After updating you will see this error. as i have solved with this solution.

Answer 15

For outlook use following setting that is not giving error to me

SMTP server name smtp-mail.outlook.com

SMTP port 587

Answer 16

This error is very generic .It can be due to many reason such as The mail server is incorrect. Some hosting company uses mail.domainname format. If you just use domain name it will not work. check credentials host name username password if needed Check with hosting company.

<smtp from="[e-mail]">
        <!-- Uncomment to specify SMTP settings -->
        <network host="domain.com" port="25" password="Jin@" userName="[e-mail]"/>
      </smtp>
    </mailSettings>

Answer 17

In my case the web server IP was blocked on the mail server, it needs to be unblocked by your hosting company and make it whitelisted. Also, use port port 587.

Answer 18

If your mail server is Gmail (smtp.google.com), you will get this error when you hit the message limit. Gmail allows sending over SMTP up to only 2000 messages per 24 hours.

Answer 19

My original problem is about intermittent sending failures. E.g. First Send() succeeds, 2nd Send() fails, 3rd Send() succeeds. Initially I thought I wasn't disposing properly. So I resorted to using().

Anyways, later I added the UseDefaultCredentials = false, and the Send() finally became stable. Not sure why though.

Answer 20

If you are using Sendgrid and if you receive this error, it is because Basic authentication is no more allowed by sendgrid.We need to create API key and use them as NetworkCredential. username="apikey" password will be your API key Reference - https://docs.sendgrid.com/for-developers/sending-email/integrating-with-the-smtp-api

Answer 21

Prepare: 1. HostA is SMTP virtual server with default port 25 2. HostB is a workstation on which I send mail with SmtpClient and simulate unstable network I use clumsy

Case 1 Given If HostB is 2008R2 When I send email. Then This issue occurs.

Case 2 Given If HostB is 2012 or higher version When I send email. Then The mail was sent out.

Conclusion: This root cause is related with Windows Server 2008R2.