The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.

Question

 try
                MailMessage mail = new MailMessage();
                SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
                mail.From = new MailAddress("[email protected]");
                mail.To.Add("[email protected]");
                mail.Subject = "Test Mail..!!!!";
                mail.Body = "mail with attachment";
                System.Net.Mail.Attachment attachment;
                attachment = new System.Net.Mail.Attachment(@"C:\Attachment.txt");
                mail.Attachments.Add(attachment);
                SmtpServer.Port = 587;
                SmtpServer.UseDefaultCredentials = true;
                SmtpServer.Credentials = new System.Net.NetworkCredential("userid", "Password");
                SmtpServer.EnableSsl = true;
                SmtpServer.Send(mail);
Catch(Exception exception)
When i m run this part of code it throw an Ecxeption                                                          
        Given Below is the Error.. 
    The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.
Bikky Kumar

Answer 1

 try
                MailMessage mail = new MailMessage();
                SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
                mail.From = new MailAddress("[email protected]");
                mail.To.Add("[email protected]");
                mail.Subject = "Test Mail..!!!!";
                mail.Body = "mail with attachment";
                System.Net.Mail.Attachment attachment;
                attachment = new System.Net.Mail.Attachment(@"C:\Attachment.txt");
                mail.Attachments.Add(attachment);
                SmtpServer.Port = 587;
SmtpServer.UseDefaultCredentials = true;    ///Set it to false, or remove this line
                SmtpServer.Credentials = new System.Net.NetworkCredential("userid", "Password");
                SmtpServer.EnableSsl = true;
                SmtpServer.Send(mail);
Catch(Exception exception)
Given Below is the Error..      The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.
Solution:
The error might occur due to following cases.
case 1: when the password is wrong
case 2: when you try to login from some App
case 3: when you try to login from the domain other than your time zone/domain/computer (This
is the case in most of scenarios when sending mail from code)
There is a solution for each
solution for case 1: Enter the correct password.
Recomended: solution for case 2: go to
security settings at the following link https://www.google.com/settings/security/lesssecureapps and
enable less secure apps . So that you will be able to login from all apps.
solution 1 for case 3: (This might be helpful) you need to review the activity. but reviewing the activity will not be helpful due to latest security
standards the link will not be useful. So try the below case.
solution 2 for case 3: If you have hosted your code somewhere on production server and if you have access to the production server, than take remote
desktop connection to the production server and try to login once from the browser of the production server. This will add exception for login to google and you will be allowed to login from code.
But what if you don't have access to the production server. try
the solution 3
solution 3 for case 3: You have to enable
login from other timezone / ip for your google account.
to do this follow the link https://g.co/allowaccess and
allow access by clicking the continue button.
And that's it. Here you go. Now you will be able to login from any of the computer and by any means of app to your google account.
Regards,
Nabeel Arif