System.Net.Mail.SmtpException: 'The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Auth

Question

 //less security is enabled
smtp.Host = "smtp.gmail.com";
smtp.EnableSsl = true;
NetworkCredential NetworkCred = new NetworkCredential("pk.uzikhan@gmail.com", "*********");
smtp.UseDefaultCredentials = true;
smtp.Credentials = NetworkCred;
smtp.Port = 587;
smtp.Send(mm);
//       ViewBag.Message = "Email sent.";
return Ok("email sent");

Answer 1

I think you could read the official document about SmtpClient:https://learn.microsoft.com/en-us/dotnet/api/system.net.mail.smtpclient.credentials?view=netframework-4.8#system-net-mail-smtpclient-credentials

you could see the Remark on the "SmtpClient.Credentials Property":

To use your default network credentials, you can set the UseDefaultCredentials to true instead of setting this property.

Make sure you are using the correct account and password.Also, don't forget set your google account as below:

I tested with the codes:

            MailMessage mail = new MailMessage();
            SmtpClient SmtpServer = new SmtpClient();
            mail.From = new MailAddress(emailaddress);
            mail.To.Add(emailaddress);
            mail.Subject = "Hi";
            mail.Body = "Doc";
            SmtpServer.UseDefaultCredentials = false;            
            NetworkCredential NetworkCred = new NetworkCredential(emailaddress, password);
            SmtpServer.Credentials = NetworkCred;
            SmtpServer.EnableSsl = true;
            SmtpServer.Port = 587;            
            SmtpServer.Host = "smtp.gmail.com";
            SmtpServer.Send(mail);
            

The result:

You could see the explain of the error message in this case: System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated