blocks|key|190470|text|您的SMTP库是否支持加密连接？邮件服务器可能期望安全的TLS连接，因此在没有TLS握手的情况下关闭连接|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190471|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

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

blocks|key|4696306|text|试试这个:这是我用来向多个用户发送电子邮件的代码。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4696307|+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%2B%2B)
++++++++++++++++++++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+"+%2B+exp;
++++++++++++}
++++++++}
++++}|code-block|syntax|javascript|4696308|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

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

<pre><code> 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 &lt; 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;
 }
 }
 }
</code></pre>

blocks|key|190519|text|将端口从465更改为587，即可正常工作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190520|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Change port from 465 to 587 and it will work.

blocks|key|190538|text|您可能还必须更改您的Gmail帐户上的“不太安全的应用程序”设置。EnableSsl，使用端口587并启用“安全性较低的应用程序”。如果你在谷歌上搜索不太安全的应用程序部分，会有谷歌帮助页面将你链接到你的账户的页面。这就是我的问题，但多亏了上面所有的答案，现在一切都正常了。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190539|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

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.

blocks|key|190558|text|如果您在同一机器上使用SMTP服务器，并且您的SMTP绑定到一个IP地址，而不是"Any+Assigned“，则可能会失败，因为它正在尝试使用SMTP当前未使用的IP地址(如127.0.0.1)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190559|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

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.

blocks|key|190607|text|如果以上所有解决方案都不适用于您，请尝试将以下文件更新到您的服务器(通过发布，我的意思是，在此之前的构建会很有帮助)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190608|bin->+projectname.dll+|code-block|syntax|javascript|190609|更新后，您将看到此错误。就像我用这个解决方案解决的那样。|190610|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

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).

<pre><code>bin-&gt; projectname.dll 
</code></pre>

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

blocks|key|4696446|text|为了提升jocull在评论中提到的内容，我正在做这篇文章中提到的所有事情，并删除...因为我的是在一个循环中一遍又一遍地运行；在第一次通过循环之后，它有时会失败。总是在第一次循环中起作用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4696447|需要说明的是:该循环包括创建SmtpClient，然后使用正确的数据执行.Send。SmtpClient是在一个try/catch块中创建的，目的是捕获错误并确保对象在循环结束前被销毁。|4696448|在我的例子中，解决方案是确保在循环中的每一次之后都释放SmtpClient+(通过using()语句或手动dispose)。即使SmtpClient对象在循环中被隐式销毁，.NET似乎也会留下一些东西，以便与下一次尝试发生冲突。|4696449|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

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.

blocks|key|190679|text|对于outlook的使用，以下设置不会给我错误|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190680|SMTP服务器名称smtp-mail.outlook.com|190681|SMTP端口587|190682|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

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

SMTP server name smtp-mail.outlook.com

SMTP port 587

blocks|key|4696492|text|我已经尝试了上面的所有答案，但Office+365帐户仍然出现此错误。当允许不太安全的应用程序时，代码似乎在谷歌账户和smtp.gmail.com上运行得很好。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4696493|还有什么其他的建议我可以尝试一下吗？|4696494|下面是我使用的代码|4696495|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);
}|code-block|syntax|javascript|4696496|更新以及我是如何解决它的：|4696497|通过将Smtp客户端更改为Mailkit解决了此问题。由于安全问题，微软现在不建议使用System.Net.Mail+Smtp客户端，而应使用MailKit。使用Mailkit给了我更清晰的错误消息，我可以理解寻找问题的根本原因(许可证问题)。你可以通过下载一个Nuget包来获取邮件包。|offset|length|4696498|有关更多信息，请阅读有关Smtp客户端的文档：https://docs.microsoft.com/es-es/dotnet/api/system.net.mail.smtpclient?redirectedfrom=MSDN&view=netframework-4.7.2|4696499|下面是我如何使用MailKit实现SmtpClient|4696500|++++++++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);
++++++++}|4696501|entityMap|0|LINK|mutability|MUTABLE|url|https://www.nuget.org/packages/MimeKit/|1|https://docs.microsoft.com/es-es/dotnet/api/system.net.mail.smtpclient?redirectedfrom=MSDN&view=netframework-4.7.2^0|0|0|0|0|0|3N|5|0|0|N|36|1|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|17|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|18|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|19|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|1A|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1B|8|@]|9|@$O|1C|P|1D|1|1E]]|A|$]]|$1|Q|3|R|5|6|7|1F|8|@]|9|@$O|1G|P|1H|1|1I]]|A|$]]|$1|S|3|T|5|6|7|1J|8|@]|9|@]|A|$]]|$1|U|3|V|5|H|7|1K|8|@]|9|@]|A|$I|J]]|$1|W|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]]|X|$Y|$5|Z|10|11|A|$12|13]]|14|$5|Z|10|11|A|$12|15]]]]

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

<pre><code>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);
}
</code></pre>

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 <a href="https://www.nuget.org/packages/MimeKit/" rel="noreferrer">Nuget</a> Package. 

Read documentation about Smtp Client for more information:
<a href="https://docs.microsoft.com/es-es/dotnet/api/system.net.mail.smtpclient?redirectedfrom=MSDN&amp;view=netframework-4.7.2" rel="noreferrer">https://docs.microsoft.com/es-es/dotnet/api/system.net.mail.smtpclient?redirectedfrom=MSDN&amp;view=netframework-4.7.2</a>

Here is how I implemented SmtpClient with MailKit

<pre><code> 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);
 }
</code></pre>

blocks|key|190747|text|这个错误是非常普遍的.It可能是由于许多原因，如邮件服务器不正确。有些托管公司使用mail.domainname格式。如果您只使用域名，它将无法工作。检查凭证、主机名、用户名、密码，如果需要，请与托管公司联系。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190748|<smtp+from="info@india.uu.com">
++++++++
++++++++<network+host="domain.com"+port="25"+password="Jin@"+userName="info@india.xx.com"/>
++++++</smtp>
++++</mailSettings>|code-block|syntax|javascript|190749|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

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.

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

blocks|key|190770|text|在我的案例中，客户忘记在他们的SMTP设置中添加新的IP地址。在设置smtp的服务器中打开IIS+6.0，右键单击Smtp虚拟服务器，选择属性，访问选项卡，单击连接，添加新服务器的IP地址。然后单击Relay，同时添加新服务器的IP地址。这解决了我的问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190771|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

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.

blocks|key|4696591|text|将端口号从465更改为587|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4696592|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Change your port number to 587 from 465

blocks|key|190812|text|删除|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190813|client.UseDefaultCredentials+=+false;+|code-block|syntax|javascript|190814|似乎为我解决了这个问题。|190815|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

removing 

<pre><code>client.UseDefaultCredentials = false; 
</code></pre>

seemed to solve it for me.

blocks|key|190850|text|在我的案例中，web服务器IP在邮件服务器上被阻止，它需要由您的托管公司解除阻止，并将其列入白名单。另外，使用端口587。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190851|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

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.

blocks|key|4696672|text|如果您的邮件服务器是Gmail+(smtp.google.com)，当您达到消息限制时，您将收到此错误。Gmail允许每24小时通过SMTP最多发送2000条消息。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4696673|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

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.

blocks|key|190981|text|我在使用smtp.office+e365.com，使用带有SSL的端口587时遇到了这个问题。我能够使用portal.office.com登录到该帐户，并且我可以确认该帐户具有许可证。但是当我启动代码发送电子邮件时，我一直收到net_io_connectionclosed错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|190982|我花了一些时间才弄清楚，但Exchange管理员找到了罪魁祸首。我们使用的是O365，但Exchange+server处于混合环境中。尽管我们尝试使用的帐户已同步到Azure+AD，并且拥有有效的O365许可证，但由于某些原因，邮箱仍然驻留在混合Exchange服务器上-而不是Exchange+online。在exchange管理员使用"+move+-+mailbox“命令将邮箱从混合exchange服务器移动到o365之后，我们可以使用代码通过O365发送电子邮件。|190983|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

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.

blocks|key|4696792|text|我最初的问题是关于间歇性的发送失败。例如，第一个Send()成功，第二个Send()失败，第三个Send()成功。最初，我认为我没有正确处理。所以我求助于using()。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4696793|无论如何，后来我添加了UseDefaultCredentials+=+false，Send()最终变得稳定起来。不过，我不知道为什么。|4696794|entityMap^0|O|6|10|6|1C|6|25|7|0|B|T|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|I|$]]

My original problem is about intermittent sending failures. E.g. First <code>Send()</code> succeeds, 2nd <code>Send()</code> fails, 3rd <code>Send()</code> succeeds. Initially I thought I wasn't disposing properly. So I resorted to <code>using()</code>.
Anyways, later I added the <code>UseDefaultCredentials = false</code>, and the Send() finally became stable.
Not sure why though.

blocks|key|191129|text|如果您正在使用Sendgrid，并且收到此错误，则是因为sendgrid.We不再允许基本身份验证，因此需要创建API+key并将其用作NetworkCredential。username="apikey“密码将作为您的API密钥引用-+https://docs.sendgrid.com/for-developers/sending-email/integrating-with-the-smtp-api|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|191130|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.sendgrid.com/for-developers/sending-email/integrating-with-the-smtp-api^0|3B|2C|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

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=&quot;apikey&quot; password will be your API key
Reference - <a href="https://docs.sendgrid.com/for-developers/sending-email/integrating-with-the-smtp-api" rel="nofollow noreferrer">https://docs.sendgrid.com/for-developers/sending-email/integrating-with-the-smtp-api</a>

blocks|key|191215|text|特定于Outlook+Mailer的答案|type|unstyled|depth|inlineStyleRanges|entityRanges|data|191216|var+SmtpClient+=+new+SmtpClient{
++++++++++++++++DeliveryMethod+=+SmtpDeliveryMethod.Network,
++++++++++++++++Credentials+=+new+System.Net.NetworkCredential("email",+"password"),
++++++++++++++++Port+=+587,
++++++++++++++++Host+=+"smtp.office365.com",
++++++++++++++++EnableSsl+=+true+}|code-block|syntax|javascript|191217|https://admin.exchange.microsoft.com/#/settings+->点击邮件流|offset|length|191218|TLS检查-启用对传统->客户端的使用|191219|->保存|191220|​|191221|📷|atomic|191222|191223|entityMap|0|LINK|mutability|MUTABLE|url|https://admin.exchange.microsoft.com/#/settings|1|IMAGE|IMMUTABLE|imageUrl|https://ask.qcloudimg.com/http-save/yehe-900000/c4a1795a3aa83b1d8083846d38af778f.png|imageAlt^0|0|0|0|1B|0|0|0|0|0|0|1|1|0|0^^$0|@$1|2|3|4|5|6|7|18|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|19|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1A|8|@]|9|@$I|1B|J|1C|1|1D]]|A|$]]|$1|K|3|L|5|6|7|1E|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1F|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1G|8|@]|9|@]|A|$]]|$1|Q|3|R|5|S|7|1H|8|@]|9|@$I|1I|J|1J|1|1K]]|A|$]]|$1|T|3|P|5|6|7|1L|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|1M|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|11]]|12|$5|13|Y|14|A|$15|16|17|-4]]]]

Answer Specific to Outlook Mailer
<pre><code>var SmtpClient = new SmtpClient{
 DeliveryMethod = SmtpDeliveryMethod.Network,
 Credentials = new System.Net.NetworkCredential(&quot;email&quot;, &quot;password&quot;),
 Port = 587,
 Host = &quot;smtp.office365.com&quot;,
 EnableSsl = true }
</code></pre>
<a href="https://admin.exchange.microsoft.com/#/settings" rel="noreferrer">https://admin.exchange.microsoft.com/#/settings</a>
-&gt; Click on Mail Flow
-&gt; Check - Turn on use of legacy TLS clients
-&gt; Save
<a href="https://i.stack.imgur.com/wikq4.png" rel="noreferrer"><img src="https://i.stack.imgur.com/wikq4.png" alt="enter image description here" /></a>

blocks|key|191286|text|把这个放在我的方法的开头为我解决了这个问题|type|unstyled|depth|inlineStyleRanges|entityRanges|data|191287|System.Net.ServicePointManager.SecurityProtocol+=+System.Net.SecurityProtocolType.Tls12|code-block|syntax|javascript|191288|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Putting this at the beginning of my method fixed this for me
<pre><code>System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12
</code></pre>

blocks|key|191334|text|我最近不得不在我们所有的应用程序上设置新的邮件设置，并且在多个项目中遇到了这个错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|191335|我的解决方案是在我的一些项目上将目标框架更新为较新的版本。|191336|我还有一个ASP.net网站项目，其中仅更新目标框架是不够的，我还必须在web.config+<httpRuntime++targetFramework="4.8"/>中添加以下代码|offset|length|style|CODE|191337|entityMap^0|0|0|1B|11|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|N|8|@$F|O|G|P|H|I]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|K|$]]

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 <code>&lt;httpRuntime targetFramework=&quot;4.8&quot;/&gt;</code>

I am using the <code>SmtpClient</code> library to send emails using the following:

<pre><code>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");
</code></pre>

The code works fine in my test environment, but when I use production SMTP servers, the code fails with an <code>SmtpException</code> "Failure sending mail." with an inner <code>IOException</code> "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.

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

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我正在使用SmtpClient库通过以下方式发送电子邮件：SmtpClient client = new SmtpClient();client.Host = "hostname";client.Port = 465;client.DeliveryMethod = SmtpDeliveryMethod.Network;...

问SmtpException:无法从传输连接读取数据: net_io_connectionclosed
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