If a connection to the customers server cannot be established (timeout of approx 10 seconds), or their server answers with a 400 SMTP error for some reason. The Continuity Service gets activated immediately (if already activated of course in the customers Control Panel). The mails are then sent to another server called the portal server. This server administers the POP accounts, the access by IMAP and even the automatic checking service which monitors the status of the customer's server.