You find that on your wordpress based site you see a message saying “Briefly unavailable for scheduled maintenance” . This can be fixed by removing a file in your root folder. This issue can come from a a wordpress upgrade or a wordpress plugin upgrade
Mail stops flowing in exchange 2007 or exchange 2010, in event viewer you see the event id 15006 and The Microsoft Exchange Transport service is rejecting message submissions because the available disk space has dropped below the configured threshold. This can be fixed by moving the mail.que file.