class Net::SMTP::Response

def exception_class

based on the reply code of the response
Determines whether there was an error and raises the appropriate error 
def exception_class
  case @status
  when /\A4/  then SMTPServerBusy
  when /\A50/ then SMTPSyntaxError
  when /\A53/ then SMTPAuthenticationError
  when /\A5/  then SMTPFatalError
  else             SMTPUnknownError
  end
end