Hi Roy, you might have found this answer but if not this is from the FAQ guide
Question: My server went down for a short period of time – what happened to
our company’s messages?
Answer: Symantec Hosted Mail Security attempts to connect to all the servers that are configured for your Domain in the order designated in the Preference column, from the lowest number to the highest number. If Symantec Hosted Mail Security cannot establish a connection with any of your email server(s), it
will deliver a temporary failure message to the sending email server. When this occurs, the sending email server will usually attempt to redeliver the message again.
Most email servers are set to keep trying to deliver the message for an extended period of time before they finally stop and permanently fail the message. This frequency varies by mail server configuration.
If you enable the FailSafe Disaster Recovery feature, Symantec Hosted Mail Security will spool up to 1 GB of your messages up to 5 days if Symantec is unable to deliver your messages because your mail servers are down.