InboxGuardian has the ability to match received email against email blacklists (known a DNS blacklists) to further guard against spam.
What is a DNS blacklist?
DNS blacklists are lists of domains that are known to originate Spam.
How does InboxGuardian use DNS blacklists?
By default InboxGuardian has three blacklists set up for you. When email is being received the sender IP is run through the listed blacklists to see if there is a match. If there is a match then the email in question is set for deletion and moved to the deleted tab.
Can valid emails be trapped by blacklists?
Occassionally valid emails can be caught by blacklists. There are hundreds of available blacklists to choose from and some are poorly maintained. These are normally the reason behind valid emails being flagged as spam. However, InboxGuardian moves all deleted emails to a tab marked "deleted". This feature keeps deleted email for a user defined number of days. So even if valid emails are set to be deleted because they match a blacklist you don't lose the email. They can be retrieved from the "deleted" tab.