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

把它放在我的方法的开头为我解决了这个问题

System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12

Answer 3

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 4

将端口从 465 更改为 587 即可。

Answer 5

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 = "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 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 = "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);
        }

Answer 6

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 7

Answer Specific to Outlook Mailer

var SmtpClient = new SmtpClient{
                DeliveryMethod = SmtpDeliveryMethod.Network,
                Credentials = new System.Net.NetworkCredential("email", "password"),
                Port = 587,
                Host = "smtp.office365.com",
                EnableSsl = true }

https://admin.exchange.microsoft.com/#/settings -> Click on Mail Flow

-> Check - Turn on use of legacy TLS clients

-> Save

Answer 8

removing

client.UseDefaultCredentials = false; 

seemed to solve it for me.

Answer 9

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 10

如果您在同一个盒子上使用 SMTP 服务器，并且您的 SMTP 绑定到 IP 地址而不是“任何已分配”，则它可能会失败，因为它正在尝试使用 SMTP 当前无法工作的 IP 地址（如 127.0.0.1）上。

Answer 11

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 12

将端口号从 465 更改为 587

Answer 13

I got the same problem with the .NET smtp client + office 365 mail server: sometimes mails were sent successfully, sometimes not (intermittent sending failures).

The problem was fixed by setting the desired TLS version to 1.2 only . The original code (which started to fail in the middle of the year 2021 - BTW) was allowing TLS 1.0, TLS 1.1 and TLS 1.2.

Code (CLI/C++)

    int tls12 = 3072; // Tls12 is not defined in the SecurityProtocolType enum in CLI/C++ / ToolsVersion="4.0"  
    System::Net::ServicePointManager::SecurityProtocol = (SecurityProtocolType) tls12;

(note: the problem was reproduced and fixed on a Win 8.1 machine)

Answer 14

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 15

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 16

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 17

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 18

I recently had to set new mail settings on all our applications and encountered this error on multiple projects.

The solution for me was to update the target framework to a newer version on some of my projects.

I also had an ASP.net website project where updating the target framework wasn't enough I also had to add the following code to the web.config <httpRuntime targetFramework="4.8"/>

Answer 19

Since Jan 22 2022, Google has increased the TLS version requirements Also Microsoft has revoked the support for TLS 1.0 and TLS 1.1 for the earlier versions of the .NET framework than 4.6.

So we can fix the issue one of the below 2 solutions.

1.By Adding some other Protocols before creating the smtp client >> ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

2.You just need to update the .NET framework version to 4.6 or higher to fix the issue.

Answer 20

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 21

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 22

//Try this out... Port 465 is not there anymore for use
     string emailFrom = "sender email here";
    string recieverEmail ="reciever email here";
            string subject = "subject here";
            string body = "message here";
            MailMessage maile = new MailMessage(emailFrom, recieverEmail, subject,body);
            SmtpClient client = new SmtpClient("smtp.gmail.com", 587);
            client.Credentials = new System.Net.NetworkCredential("senders gmail account username here", "senders gmail account password here");
            client.EnableSsl = true;

Answer 23

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

SMTP server name smtp-mail.outlook.com

SMTP port 587

Answer 24

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="info@india.uu.com">
        <!-- Uncomment to specify SMTP settings -->
        <network host="domain.com" port="25" password="Jin@" userName="info@india.xx.com"/>
      </smtp>
    </mailSettings>

Answer 25

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 26

My original problem is about intermittent sending failures. Eg 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 27

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

There are two solutions. First solution is for app level (deployment required) and second one is for machine level (especially if you use an out-of-the-box / off-the-shelf app)

When we checked the exception, we saw that the protocol is "ssl|tls" depriciated pair.

Since we don't want to deploy, we prefer machine level change (Solution 2).

On August 18, Microsoft announced that they will disable Transport Layer Security (TLS) 1.0 and 1.1 connections to Exchange Online “in 2022.” https://office365itpros.com/2021/08/19/exchange-online-to-introduce-legacy-smtp-endpoint-in-2022/

Firstly let's check the network (Anything prevents your email sent request? firewall, IDS, etc.)

By using PowerShell check Transport Layer Security protocols

[Net.ServicePointManager]::SecurityProtocol

My Output : Tls, Tls11, Tls12

Test SMTP Authentication over TLS

$HostName = [System.Net.DNS]::GetHostByName($Null).HostName
$Message = new-object Net.Mail.MailMessage 
$smtp = new-object Net.Mail.SmtpClient("smtp.office365.com", 587) 
$smtp.Credentials = New-Object System.Net.NetworkCredential("me@me.com", "PassMeme"); 
$smtp.EnableSsl = $true 
$smtp.Timeout = 400000  
$Message.From = "sender@me.com" 
$Message.Subject = $HostName + " PowerShell Email Test"
$Message.Body = "Email Body Message"
$Message.To.Add("receiver@me.com") 
#$Message.Attachments.Add("C:\foo\attach.txt") 
$smtp.Send($Message)

My output: There is no error message If there is any message on your output something prevents your email sent request.

If everything is ok there should be two solutions.

Solution 1:

Application Level TLS 1.2 Configuration (Optional) Application deployment required.

Explicitly choose TLS in C# or VB code:

ServicePointManager.SecurityProtocol |=  SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
System.Net.ServicePointManager.SecurityProtocol = System.Net.ServicePointManager.SecurityProtocol Or SecurityProtocolType.Tls11 Or SecurityProtocolType.Tls12

Solution 2:

Machine level TLS 1.2 .NET Framework configuration Application deployment NOT required.

Set the SchUseStrongCrypto registry setting to DWORD:00000001. You should restart the server.

For 32-bit applications on 32-bit systems or 64-bit applications on 64-bit systems), update the following subkey value:

HKEY_LOCAL_MACHINE\SOFTWARE\
   \Microsoft\.NETFramework\\<version>
      SchUseStrongCrypto = (DWORD): 00000001

For 32-bit applications that are running on x64-based systems, update the following subkey value:

HKEY_LOCAL_MACHINE\SOFTWARE\
    Wow6432Node\Microsoft\\.NETFramework\\<version>
       SchUseStrongCrypto = (DWORD): 00000001

For details "How to enable TLS 1.2 on clients" on https://docs.microsoft.com/en-us/mem/configmgr/core/plan-design/security/enable-tls-1-2-client

Answer 28

Our email service is Azure SendGrid. Our application stopped sending emails one day, and the error message was "SmtpException: Unable to receive data from the transport connection: net io connectionclosed." We discovered the problem was caused by the fact that our Pro 300K subscription had run out. Emails began to be sent when we upped our subscription.

Answer 29

I was facing the same issue with my .NET application.

ISSUE: The .NET version that I was using is 4.0 which was creating the whole mess.

REASON: The whole reason behind the issue is that Microsoft has revoked the support for TLS 1.0 and TLS 1.1 for the earlier versions of the .NET framework than 4.6.

FIX: You just need to update the .NET framework version to 4.6 or higher to fix the issue.

Answer 30

First Use Port = 587

Generally STARTTLS is required to send mail, Adding the Security Protocol Tls12 will help to resolve this issue.

Secondly test the stmp connection using powershell

$userName = 'username_here'
$password = 'xxxxxxxxx'
$pwdSecureString = ConvertTo-SecureString -Force -AsPlainText $password
$credential = New-Object -TypeName System.Management.Automation.PSCredential -ArgumentList $userName, $pwdSecureString

$sendMailParams = @{
    From = 'abc.com'
    To = 'xyz@gmail.com'
    Subject = 'Test SMTP'
    Body = 'Test SMTP'
    SMTPServer = 'smtp.server.com'
    Port = 587
    UseSsl = $true
    Credential = $credential
}

Send-MailMessage @sendMailParams

Thirdly If this send out the email, Add below code inside SmtpClient function:

System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;                            

Answer 31

Updating the .NET Framework target for the project fixed the issue for me.

Answer 32

Below is the code that work for me

System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;

            // Create the email object first, then add the properties.
            SmtpClient client = new SmtpClient()
            {
                Port = 587,
                EnableSsl = true,
                DeliveryMethod = SmtpDeliveryMethod.Network,
                //Set Credentials
                UseDefaultCredentials = false,
                Credentials = new NetworkCredential(_username, _password),
                Host = "smtp.office365.com"
            };
            client.Send(mail);

Answer 33

I have Found the Ultimate Answer to this. I've been on this error for about a week and found a nuget package that fixed this problem with smtp. You can use MailKit nuget and use it. to learn how to use it, you can just google it's name and you will find a github repository that He explained fully in Readme file.

Answer 34

System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12

its work for me. Security protocole about framework used

Answer 35

After trying all sorts of TLS/SSL/port/etc things, for me the issue was this: the username and password I was using for Credentials were not correct, apparently.

Normally our websites use a different set of credentials but this one's were different. I had assumed they were correct but apparently not.

So I'd double check my credentials if nothing else is working for you. What a precise error message!

Answer 36

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.