Type 1 DNSbl Definition- "Email software decision making based upon the identity of the
most recent, non-local mail server, that attempts to deliver email."
Examples - The most typical of this type, is the MAPS service (now subscription only)
and the broad range of other DNS block lists. Currently easily configured
into most, if not all, major email server software packages.
Analogy - Look through the keyhole on your door, and refuse to open
you door to accept packages delivered to your home,
but only from courier companies, or door knockers, where you can see who they
are, and you've had problems with them before.
Advantages - Very low overheads, very fast decision making
Disadvantages - Colateral damage to the innocent people who accidentally
chose to use a courier on your 'reject' list.
Coverage - Widely used methodology, due to it's long term inclusion into the
default setup's of many software packages.
Success - Blocks lots of junk and unwanted emails.
t2.dnsbl.net.au
Type 2 DNSbl Definition - "Email software decision making based upon all non-local identities,
of all the mail servers in the presented email's delivery path."
Examples - Some packages do carry out this method, though it seems to be not
widely supported.
Analogy - Open the door to chat with the courier, have a chat, and look through
the routing delivery docket. Pay the courier for delivering the delivery docket
to you. If the routing slip for that parcel indicates it has passed through other companies,
countries or agents that you have had no problems with in the past, then accept
and sign for the parcel. Else, if you've had problems with some of them before,
do not accept the parcel.
Advantages - Fairly low overheads, pretty quick decision making, less colateral
damage than Type 1, as you can be more selective.
Disadvantages - Some colateral damage to the innocent people who accidentally
chose to use one of the couriers on your 'reject' list.
Coverage - Not a widely used technique.
Success - Combined with Type 1 methods, should be quite successful
t3.dnsbl.net.au
Type 3 DNSbl Definition - "Email software decision making based upon further analysis of
the contents of the message itself, the routing delivery path and also the
most recent delivery agent."
Examples - Most end user client packages carry out this method.
SpamAssassin is an example of a server solution, which does. Pretty much any software that filters
based upon 'rude word' or other 'prohibited content' lists does.
Analogy - Open the door. Sign for the parcel. Pay the courier for delivering
the whole lot. For a prelim over-view, feed the parcel through your local metal
detector machine, (like at the airport) to look for bombs. Get your handy sniffer dog
to sniff for drugs and other things. If it looks sus, either send it back (paying the
courier again) or just throw it away into the trash can. If it looks OK after the
prelims, open it up, and look through all the contents. If anything matches your
list of 'rejectable reasons', then either send it back (paying the courier) or
throw it away.
Advantages - Significant overheads, can delay message delivery due to those overheads,
complicated decision making, less colateral damage than Type 1 and Type 2, as
you can be even more selective.
Disadvantages - Lowest colateral damage of all, as the 'selection criteria' is
normally under the direct control of the end user.
Coverage - Widely used technique as a 'filter' under all modern email clients. Success - Combined with Type 1 and Type 2 methods, should be very successful
