type of filter based on the internet address of an incoming
email connection. Basically, mail arrives with a hello from
another email server. The DNSBL filter is built to lookup
and compare the incoming IP address to an online listing of
undesirable IP addresses. This listing resides as a zone or
a special domain on a DNS server. If the incoming address
is blacklisted, (i.e. if a match on the IP address is found)
the connection is rejected. This process happens early in
a SMTP (Simple Mail Transport Protocol) transaction.
DNSBL (also known as an RBL or Real-time Blackhole List) is
maintained in the form of IP zone data on a particular DNS
server or hierarchy of servers. This data can be supplied
through complaints, through automated testing, through statistics,
through information derived from WHOIS information, etc. In
some cases, the zone data is compiled from all of the above.