When your mail server becomes unavailable (for example, due to a crashed server or network connectivity problems), Intelligent Spooling stores or spools your mail for later release when your server is ready.

When and How Messages are Spooled
The Spool Manager monitors connections to your mail servers and decides when to start spooling mail based on connectivity failures and the spool delay you configure. If there is sufficient mailflow through the system (about one connection every 15 seconds), and three connections fail within a 60 second window, the spool delay countdown begins. During this period, if the message filtering service receives sufficient mail flow, and every subsequent delivery attempt fails, spooling begins. When mail is spooling, messages are accepted and stored for your use. The sender will not see any error.

During spooling, messages are processed according to these guidelines:

• Messages that successfully passed through junk mail filters, virus scanning, and mail policies are spooled.
• Junk mail and virus-infected messages are not spooled -- they are quarantined according to the normal filtering mechanism. Quarantined messages are accessible even if your mail server is down.
• The Blatant Spam Blocking filter rejects the blatant spam messages as usual.
• If your spooling space runs out, spooling stops, and incoming messages are deferred with the error: 451 Can’t connect to domain.com - psmtp. Most sending servers will keep trying to send the message for 5 days.