Error: Service not available, closing transmission channel. The server response was: 4.7.0 Temporary System Problem. Try again later in ASP.Net

I am sending mass email using SMTP referencing Send Bulk (Mass) Email in ASP.Net using C# and VB.Net but it is very slow. It takes 30s to send 5 emails:( I tried using thread in sendmail function that seems fast but after sending 10-12 emails it throws exception:

An unhandled exception of type 'System.Net.Mail.SmtpException' occurred in System.dll Additional information: Service not available, closing transmission channel. The server response was: 4.7.0 Temporary System Problem. Try again later (WS). u12sm5363862pfg.146 - gsmtp

and rest of the emails remains unsent. Dont know whats wrong. Any help regarding this would be appreciated. Thanks!!