Oops, surprisingly, I can use the class method even without passing 
authentication info such as acct and passwd.


Net::SMTP.start(smtpServer) |smtp|
  smtp.sendmail( msg, from, to)
end


While I got the error as my last post after creating an smtp object 

smtp=Net::SMTP.new(smtpServer)
smtp.start(......)
smtp.sendmail( msg, from, to)