Target Skills:
Ability to describe the different levels of message filtering.
Ability to describe questions to ask before setting up filtering.
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Setting up Message Filtering
The Messaging Server gives you great flexibility in filtering messages.
We'll discuss the different levels of filtering, questions to ask before
filtering, and then cover the built in UBE (Unsolicited Bulk Email) filter
available through the Console.
Different Levels of Filtering
There are basically 3 places you can do filtering on messages as they move
through the SMTP process:
1. TCP Level
Referred to as TCP-Wrappers, these filter before the SMTP connection
is made. The allow/deny filters for smtp services discussed in the Security
Module are an example.
2. Pre-SMTP Accept
Referred to a Protocol-Level plugins, these filter before the
messaging server accepts the message into the queue on disk. The
Anti Relay plugin that ships with the product is an example.
3. Post-SMTP Accept
These filter after the message is in the queue on disk.
The UBE plugin that ships with the product is an example of this.
Questions to Ask Before Filtering
Questions you need to ask yourself before setting up filtering are:
1. Do you really need to filter messages?
Filtering consumes system resources, it can be difficult to
precisely define the characteristics of all unwanted messages, and you
run the risk of rejecting desirable messages.
2. What are the characteristics of the messages you want to
filter?
3. At what level do you want to filter?
Perhaps the best way to filter is at the protocol level, so
you don't waste your queue resources, but you have to write your own plugin.
The post-SMTP Accept level may be most desirable, even though it consumes
your queue resources, since the UBE filter plugin is already developed
and has a convenient GUI from the Console.
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