Answer
- The URI Realtime Blocklist filter verifies that the Unified Resource Identifiers (URIs) found in the email are not known to be used in spam emails. The filter performs DNS queries to the Spam URI Realtime Blocklists enabled in the product, using the URIs found in the email message body
- DNS blacklists are databases listing SMTP servers that have been used for spamming. There are quite few third party DNS blacklists available, ranging from reliable lists that have clearly outlined procedures for getting on or off the DNS blacklist to less reliable lists
- When an email is sent, it is passed through a number of SMTP servers until it reaches the final destination. The products will check all the public IPs found in the message header with the DNSBL database configured (example: ordb.org)