One of the most effective ways to control spam is by using protective software known as a filter. FIlters allow you to easily block any e-mail messages from specified addresses, domains or which have a specific subject or text from being deposited in your box.
A number of these filtering tools are available on the market and can be classified as follows:-
i. Server side spam filtering
Prevents the spam from reaching your box.
ii. Client side spam filtering
Removes the spam from your mailbox before you have read it.
iii. DNS Black Hole List
DNSBLs filters spam by using Domain Name Service (DNS) records as a database of policies relating to either an IP address or domain name. The database is then used to decide whether or not to accept (or label) e-mail.
Lists of IP addresses, domain names, e-mail addresses, content of the headers and even content of the message or some combination of these types can be used to identify and block spam. However the available blacklists can be unverified and their criteria for listing may not be clear.