If you need to shut down you server and you need to notify your users of this you can send a message to there screen notifying them of a server reboot or shutdown. This article covers windows and unix /linux.
While migrating from exchange 2003 to exchange 2010 you see mail queus building up between the exchange 2010 server send mail to exchange 2003. The error yuo see is Exchange 2010 Queue 451 4.4.0 Primary target IP address responded with: “451 5.7.3 Cannot achieve Exchange Server authentication. Solution here.