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

 
Hi,
I was able to send mail with no trouble before few days.
Then due to some reason, i Formatted pc &  re-installed visual studio & IIS
Now when i try to send mail, i get this error.

The SMTP server requires a secure connection or the client was not authenticated.In my web.config file i write
<mailSettings>
      <smtp from="myemail@gmail.com">
        <network host="smtp.gmail.com" port="587" userName="myemail" password="xxxx" defaultCredentials="false"/>
      </smtp>
    </mailSettings>
  </system.net>

 
And on SEND MAIL Button click event I write ( CODE BEHIND)

 
  Dim smtp As Ne ...