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The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required

my website uses gmail smtp to send out emails. Nothing has been changed recently and all of a sudden it has stopped today. No code or password has been changed. This seems to be a common issue and many people suggest the same solution, unfortunately those didn't work for me today.
My SMTP code
using (var mail = new MailMessage())
{
const string smtp = "smtp.gmail.com";
const int port = 587;
var loginInfo = new NetworkCredential(account, password);
mail.From = new MailAddress(account);
mail.To.Add(new MailAddress(toAddress));
mail.Subject = subject;
mail.Body = message.ToString();
mail.IsBodyHtml = true;
try
{
using (var smtpClient = new SmtpClient(smtp, port))
{
smtpClient.EnableSsl = KnownKeys.EnableSSL;
smtpClient.UseDefaultCredentials = false;
smtpClient.Credentials = loginInfo;
smtpClient.Send(mail);
}
}
finally
{
//dispose the client
mail.Dispose();
}
}
What I've tried
Resetting password
Setting smtpClient.TargetName = "STARTTLS/smtp.gmail.com";
Double checked allow unsecure apps
Verified Enabled SSL is on
Tried port 465

The problem is that you are running the application behind a firewall e.g Sophos so its blocking outbound communication from the application

How to send mail gmail via smtp?

I'm receiving an exception when trying to execute this mail-sending method.
{"Syntax error, command unrecognized. The server response was: "}
my code:
public async static Task SendExceptionMail(Exception e)
{
try
{
//TODO: fill..
var message = new MailMessage();
message.To.Add("other_email_than_my_email#gmail.com");
message.From = new MailAddress("my_email_in_gmail#gmail.com");
message.Subject = "Server Exception Occured";
StringBuilder sb = new StringBuilder();
sb.AppendLine("Exception occured. Stack trace:");
sb.AppendLine(e.StackTrace);
sb.AppendLine("");
sb.AppendLine("Time: " + DateTime.UtcNow);
message.Body = sb.ToString();
message.IsBodyHtml = false;
message.BodyEncoding = UTF8Encoding.UTF8;
using (var smtpClient = new SmtpClient())
{
smtpClient.Credentials = new System.Net.NetworkCredential("my_email_in_gmail#gmail.com", "very_very_complicated_password_with_numbers_and_signs");
smtpClient.Host = "smtp.gmail.com";
smtpClient.Port = 465;
smtpClient.EnableSsl = true;
smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
//smtpClient.UseDefaultCredentials = false;
await smtpClient.SendMailAsync(message);
}
}
catch (Exception ex)
{
Console.Write(ex.StackTrace);
}
}
In my Gmail account, I allowed IMAP and POP in the settings tab.
What I've tried:
Changing the port to 587 and 25. this time I'm getting The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at
commenting/uncommenting the UseDefaultCredentials line, the DeliveryMethod properties
commenting/uncommenting the IsBodyHtml and BodyEncoding properties

I suggest you to follow this - Sending email in .NET through Gmail and Cannot send mail from certain server
-First Check whether "Allowing less secure apps to access your account" is enable in google account setting then check with below code:
using (var smtpClient = new SmtpClient())
{
smtpClient.Credentials = new System.Net.NetworkCredential("my_email_in_gmail#gmail.com", "very_very_complicated_password_with_numbers_and_signs");
smtpClient.Host = "smtp.gmail.com";
smtpClient.Port = 587; // Google smtp port
smtpClient.EnableSsl = true;
smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
smtpClient.UseDefaultCredentials = false;// disable it
/// Now specify the credentials
smtpClient.Credentials = new NetworkCredential(message.From.Address, fromPassword)
await smtpClient.SendMailAsync(message);
}
There may be some firewall issue .
References:
SendEmail in ASP.net shows me Syntax error, command unrecognized. The server response was: Dovecot ready
Syntax error, command unrecognized. The server response was ''
Hope this help.

Not able to send email to other domains

I have some code that can only work with my SMTP host domains but not others. It seems to send, but it doesn't actually send anything and it also doesn't throw any exceptions.
SmtpClient mailClient = new SmtpClient();
mailClient.UseDefaultCredentials = false;
mailClient.Port = 587;
mailClient.EnableSsl = false;
mailClient.DeliveryMethod = SmtpDeliveryMethod.Network;
mailClient.Host = "mail.smtpserver.com";
System.Net.NetworkCredential cred = new System.Net.NetworkCredential("mail#smtpserver.com", "password");
mailClient.Credentials = cred;
MailMessage message = new MailMessage();
message.From = new MailAddress("mail#smtpserver.com");
message.To.Add("mail#remotedomain.com");
message.Subject = "Subject";
message.Body = "Body";
mailClient.Timeout = 200000;
mailClient.Send(message);
I think the problem is not checking NetworkCredential...

I would suggest you to try to send a testing mail with the "other" smtp server using telnet to make sure it works.
your smtp port is not the default one so maybe it is blocked by your firewall.

Generally, if you're using port 587 then that is SSL secured SMTP - so you need to change your code so that uses it when establishing the connection:
mailClient.EnableSsl = true;

How to send Mail in asp.net from windows mail server

I have tried sending mail from gmail and other share hosting mail server and its works as expected
But Now my requirement is sending mail from windows mail server.whenever i try to send mail it shows gives me error failure sending mail.
SmtpClient client = new SmtpClient();
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.Host = "us2.webmail.mailhostbox.com";
string fromaddress = "info#csi.com";
client.Port = 25;
//client.Host = "mail.csisite.com";
// setup Smtp authentication
System.Net.NetworkCredential credentials =
new System.Net.NetworkCredential("info#csisite.com", "password");
//client.UseDefaultCredentials = true;
client.Credentials = credentials;
client.EnableSsl = false;
try
{
//mail For Customer
MailMessage msgcustomer = new MailMessage();
msgcustomer.From = new MailAddress(fromaddress);
msgcustomer.To.Add(new MailAddress(EmailID));
msgcustomer.Subject = "Thank you for writing to us" ;
msgcustomer.IsBodyHtml = true;
msgcustomer.Body = "Test Mail";
client.Send(msgcustomer)

Send Mail:
var smtp = new System.Net.Mail.SmtpClient();
{
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.EnableSsl = true;
smtp.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
smtp.Credentials = new NetworkCredential(fromAddress, fromPassword);
smtp.Timeout = 20000;
}
// Passing values to smtp object
smtp.Send(fromAddress, toAddress, subject, body);

using System.Net.Mail
MailMessage msgMail = new MailMessage();
MailMessage myMessage = new MailMessage();
myMessage.From = new MailAddress("sender's email","sender`s name and surname");
myMessage.To.Add("recipient's email");
myMessage.Subject = "Subject";
myMessage.IsBodyHtml = true;
myMessage.Body = "Message Body";
SmtpClient mySmtpClient = new SmtpClient();
System.Net.NetworkCredential myCredential = new System.Net.NetworkCredential("email", "password");
mySmtpClient.Host = "your smtp host address";
mySmtpClient.UseDefaultCredentials = false;
mySmtpClient.Credentials = myCredential;
mySmtpClient.ServicePoint.MaxIdleTime = 1;
mySmtpClient.Send(myMessage);
myMessage.Dispose();
updated user is getting the following error
{"A socket operation was attempted to an unreachable network 208.91.199.230:25"} and Error Code is 10051
The Error 10051 is a socket error that appears when the target network or host server can not be reached. The error could have occurred due to an erroneous configuration of the router, Internet disconnection, or a blocking action by a firewall. This error has occurred mostly in attempts to generate Internet Control Message Protocol (ICMP) and Simple Mail Transfer Protocol (SMTP) connections with wrong configurations. This error also indicates that the Windows software employed is configured to communicate to a router, since the 10065 error appears instead if Windows is not set to link to a router.
you should make ensure that the network path is correct, that the computer is not running two software firewalls and that the targeted computer is not turned off.

Use This Code for sending mail this is tested code.
public static void SendMailMessage(string from, string to, string bcc, string cc, string subject, string body)
{
// Instantiate a new instance of MailMessage
MailMessage mMailMessage = new MailMessage();
// Set the sender address of the mail message
mMailMessage.From = new MailAddress(from);
// Set the recepient address of the mail message
mMailMessage.To.Add(new MailAddress(to));
// Check if the bcc value is null or an empty string
if ((bcc != null) && (bcc != string.Empty))
{
// Set the Bcc address of the mail message
mMailMessage.Bcc.Add(new MailAddress(bcc));
} // Check if the cc value is null or an empty value
if ((cc != null) && (cc != string.Empty))
{
// Set the CC address of the mail message
mMailMessage.CC.Add(new MailAddress(cc));
} // Set the subject of the mail message
mMailMessage.Subject = subject;
// Set the body of the mail message
mMailMessage.Body = body;
// Set the format of the mail message body as HTML
mMailMessage.IsBodyHtml = true;
// Set the priority of the mail message to normal
mMailMessage.Priority = MailPriority.Normal;
// Instantiate a new instance of SmtpClient
SmtpClient mSmtpClient = new SmtpClient();
// Send the mail message
mSmtpClient.Host = "smtp.gmail.com"; //Or Your SMTP Server Address
mSmtpClient.Credentials = new System.Net.NetworkCredential
("YourEmail#gmail.com", "Password");
//Or your Smtp Email ID and Password
mSmtpClient.EnableSsl = true;
mSmtpClient.Send(mMailMessage);
}

Note: I have been trying to send email through my ISP mail server with no success - following all the suggestions that I could find on these forums. I just discovered that it would work fine if I did not attempt to alter the default credentials setting. If I included either: client.UseDefaultCredentials = true; or client.UseDefaultCredentials = false; in my code, then I would get a login failure message from the server. By not including either, it worked fine.

Gmail: How to send an email programmatically

Possible Exact Duplicate: Sending Email in C#.NET Through Gmail
Hi,
I'm trying to send an email using gmail:
I tried various examples that I found on this site and other sites but I always get the same error:
Unable to connect to the remote server -- > System.net.Sockets.SocketException: No connection could be made because the target actively refused it 209.85.147.109:587
public static void Attempt1()
{
var client = new SmtpClient("smtp.gmail.com", 587)
{
Credentials = new NetworkCredential("MyEmailAddress#gmail.com", "MyPassWord"),
EnableSsl = true
};
client.Send("MyEmailAddress#gmail.com", "some.email#some.com", "test", "testbody");
}
Any ideas?
UPDATE
More details.
Maybe I should say what other attempts I made that gave me the same error:
(Note when i didn't specify a port it tryed port 25)
public static void Attempt2()
{
var fromAddress = new MailAddress("MyEmailAddy#gmail.com", "From Name");
var toAddress = new MailAddress("MyEmailAddy#dfdf.com", "To Name");
const string fromPassword = "pass";
const string subject = "Subject";
const string body = "Body";
var smtp = new SmtpClient
{
Host = "smtp.gmail.com",
Port = 587,
EnableSsl = true,
DeliveryMethod = SmtpDeliveryMethod.Network,
UseDefaultCredentials = false,
Credentials = new NetworkCredential(fromAddress.Address, fromPassword)
};
using (var message = new MailMessage(fromAddress, toAddress)
{
Subject = subject,
Body = body
}
) { smtp.Send(message); }
}
public static void Attempt3()
{
MailMessage mail = new MailMessage();
mail.To.Add("MyEmailAddy#dfdf.com");
mail.From = new MailAddress("MyEmailAddy#gmail.com");
mail.Subject = "Email using Gmail";
string Body = "Hi, this mail is to test sending mail" +
"using Gmail in ASP.NET";
mail.Body = Body;
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Credentials = new System.Net.NetworkCredential
("MyEmailAddy#gmail.com", "pass");
smtp.EnableSsl = true;
smtp.Send(mail);
}

I'm using the following code:
SmtpClient sc = new SmtpClient("smtp.gmail.com");
NetworkCredential nc = new NetworkCredential("username", "password");//username doesn't include #gmail.com
sc.UseDefaultCredentials = false;
sc.Credentials = nc;
sc.EnableSsl = true;
sc.Port = 587;
try {
sc.Send(mm);
} catch (Exception ex) {
EventLog.WriteEntry("Error Sending", EventLogEntryType.Error);
}

With the following code, it will work successfully.
MailMessage mail = new MailMessage();
mail.From = new MailAddress("abc#mydomain.com", "Enquiry");
mail.To.Add("abcdef#yahoo.com");
mail.IsBodyHtml = true;
mail.Subject = "Registration";
mail.Body = "Some Text";
mail.Priority = MailPriority.High;
SmtpClient smtp = new SmtpClient("smtp.gmail.com", 587);
//smtp.UseDefaultCredentials = true;
smtp.Credentials = new System.Net.NetworkCredential("xyz#gmail.com", "<my gmail pwd>");
smtp.EnableSsl = true;
//smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
smtp.Send(mail);
But, there is a problem with using gmail. The email will be sent successfully, but the recipient inbox will have the gmail address in the 'from address' instead of the 'from address' mentioned in the code.
To solve this, please follow the steps mentioned at the following link.
http://karmic-development.blogspot.in/2013/10/send-email-from-aspnet-using-gmail-as.html
before following all the above steps, you need to authenticate your gmail account to allow access to your application and also the devices. Please check all the steps for account authentication at the following link:
http://karmic-development.blogspot.in/2013/11/allow-account-access-while-sending.html

Here is my connection resource to connect to Gmail from Java
<!-- Java Mail -->
<Resource name="mail/MailSession" auth="Container" type="javax.mail.Session"
mail.debug="true"
mail.transport.protocol="smtp"
mail.smtp.host="smtp.gmail.com"
mail.smtp.user="youremail#gmail.com"
mail.smtp.password="yourpassword"
mail.smtp.port="465"
mail.smtp.starttls.enable="true"
mail.smtp.auth="true"
mail.smtp.socketFactory.port="465"
mail.smtp.socketFactory.class="javax.net.ssl.SSLSocketFactory"
mail.smtp.socketFactory.fallback="false"
mail.store.protocol="pop3"
mail.pop3.host="pop.gmail.com"
mail.pop3.port="995" />
Connect your Gmail account on Secure ports (465 for SMTP and 995 for POP3) and use any .NET SSL Factory available to connect securely to Gmail.

Are you sure that your GMail account is set up to allow POP/SMTP connections? It is a configurable option that you can turn on and off as you choose.

You can see my blog post here at http://codersatwork.wordpress.com/2010/02/14/sending-email-using-gmail-smtp-server-and-spring-mail/ which explains how to use spring mail for sending email via gmail smtp server.
I used java but you can see the configuration and use that in your c# code.

Try using port number 465 for SSL connection