I have a basic email web service that runs on one of my boxes. This box sends email using Amazon SES. It has sent 250,000 emails per month for the last 5 months without issue.
Suddenly, today, around lunch, the event log started spewing thousands of errors:
The remote certificate is invalid according to the validation procedure.
The email send is pretty basic. It looks like this:
using (var client = new SmtpClient(host, port))
{
client.Credentials = new NetworkCredential(username, password);
client.EnableSsl = true;
client.Send(message);
}
Host is amazon ses server, port is 25, and username and password are the smtp credentials you are given from amazon. Just to double check I created new username and password credentials and recompiled the software, but I still have the same issue.
I am decent at software but I have never worked with certs, and I have no idea how they work. All I know is the box that the emails are being sent from has a valid Digi Cert certificate and it doesn't look like anything has changed on our end.
Can someone help me resolve this or at least ask the right question?
It appears that the SSL certificate that is being presented is invalid. That could be for any number of reasons. If the hostname ('Alternate DNS' address) in the cert doesn't match the address you're connecting to, you'll get this error. The cert might not be valid according to the expiration date of the cert. The certificate issuer might no longer be trusted. The certificate may have been revoked and is in a revoked list somewhere.
What address are you trying to connect to, and what does that certificate look like?
I am trying to send out an email from my windows form application. I have seen a lot of similar posts, but none seem to work. But when I change my settings in gmail to allow less secure apps, the code works. But I don't want to make my account vulnerable for this application. Here's the code and error I get if I don't allow less secure apps.
MailMessage mail = new MailMessage("abc#gmail.com", "xyz#gmail.com", "Test Automation", "Did you receive this?");
SmtpClient client = new SmtpClient("smtp.gmail.com");
client.Port = 587;
client.Credentials = new NetworkCredential("abc#gmail.com", "password");
client.EnableSsl = true;
client.Send(mail);
MessageBox.Show("Mail Sent", "success");
An unhandled exception of type 'System.Net.Mail.SmtpException'
occurred in System.dll
Additional information: The SMTP server requires a secure connection
or the client was not authenticated. The server response was: 5.5.1
Authentication Required.
Please help!
P.S. : I created an outlook account using my gmail and when i put in the oulook server and credentials, the code works. So is it the issue with new gmail security changes? Other similar questions have their problem solved, but I keep having the same errors after trying pretty much every solution.
If you don't have 2 factor
Enable "Less Secure Apps"
https://www.google.com/settings/security/lesssecureapps
If you have 2 factor authentication
You can make an "App Password". Go to the ling below and add a custom app (just write any name you want, name not important just used for your own "bookkeeping") then use that password as the password.
client.Credentials = new NetworkCredential("From#gmail.com", "Generated Password");
https://security.google.com/settings/u/1/security/apppasswords
NOTE:
If you get "The setting you are looking for is not available for your account" then use "less secure app"
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.
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.
Putting this at the beginning of my method fixed this for me
System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12
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!
Change port from 465 to 587 and it will work.
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://learn.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);
}
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 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
removing
client.UseDefaultCredentials = false;
seemed to solve it for me.
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
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.
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.
Change your port number to 587 from 465
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)
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;
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.
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.
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.
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.
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
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"/>
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!
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;
}
}
}
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.
For outlook use following setting that is not giving error to me
SMTP server name smtp-mail.outlook.com
SMTP port 587
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>
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.
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.
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://learn.microsoft.com/en-us/mem/configmgr/core/plan-design/security/enable-tls-1-2-client
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.
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.
I have an application that needs to send e-mails. Currently, this is what I am using:
System.Net.Mail.MailMessage MyMailMessage = new System.Net.Mail.MailMessage();
MyMailMessage.From = new System.Net.Mail.MailAddress(fromemail.Text);
MyMailMessage.To.Add(toemail.Text);
MyMailMessage.Subject = subject.Text;
MyMailMessage.Body = body.Text;
System.Net.Mail.SmtpClient SMTPServer = new System.Net.Mail.SmtpClient("smtp.gmail.com");
SMTPServer.Port = 587;
SMTPServer.Credentials = new System.Net.NetworkCredential("email", "password");
SMTPServer.EnableSsl = true;
SMTPServer.Send(MyMailMessage);
Is there a simple way to send an e-mail without having to login to a server? Thank you.
GMail's SMTP server always requires authentication. You may need to setup your own server to send email without authentication.
Configure an SMTP server into your local network (behind a firewall to avoid being a spam source) and use it directly. You can create one in IIS.
There are 2 ways to achieve this:
1) Use your local smtp server (e.g. one with IIS on Win2003/2008 server) and write messages to the local pickup queue). This is possible with minimal changes.
2) You need to resolve the target smtp server. For example when you want to send an email to somebody at msn.com, you'll need to get the MX record for msn.com, e.g. something like mx1.msn.com. You can then directly connect to this SMTP server and send your email to the (local) recipient. Note that there are no built-in ways to resolve the MX-host in .NET (in the sense there are no methods on the Dns class to accomplish this) - you need to do it "manually". Also most SMTP hosts will reject connections from home/residential IP addresses.
You need an SMTP server that does not require authentication, however to stop it being a SPAM server, it needs some other kind of protection like a firewall.