A quarantine shadow account allows you to combine a number of email addresses for the purpose of viewing the quarantine of multiple accounts from a single account. Each account still maintains their own whitelist but shares a common quarantine.
Lets take a look at why this is useful. Assume your users each receive a quarantine summary one time each day. You have a user by the name of mrjones@example.com, who also receives all incoming email from sales@example.com & refunds@example.com.
With this situation mrjones@example.com will receive three email messages a day with different quarantine reports. Should he decide to monitor the quarantine online he has to login to three different accounts.
By defining sales@example.com and refunds@example.com as a quarantine shadow account of mrjones@example.com he will then receive a single daily quarantine report. The accounts remain separate in all other aspects, but will now share a common quarantine.