How to implement "Realtime Blackhole List database"

I would like to activate the Realtime Blackhole List database on my install of sendmail...

I would like to know :
1) how to activate this
2) if there might be any implications of activating this
3) if I can switch it on and off for particular domains on my server
4) If I need to pay anything for it
5) how i can test it

I have found the following information in the sendmail manual...  But it doesn't realy answer all of my questions.

I have the latest version of sendmail.

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FEATURE(`dnsbl')
This will cause sendmail to reject mail from any site in the Realtime Blackhole List database. You can specify an alternative RBL name server to contact by specifying an argument to the FEATURE. The default error message is "Mail from $&{client_addr} refused by blackhole site DOMAIN" where DOMAIN is replaced by the first argument. A second argument can be used to specify a different text. This FEATURE can be included several times to query different DNS based rejection lists, e.g., the dial-up user list (see http://maps.vix.com/dul/).

The features described above make use of the check_relay, check_mail, and check_rcpt rulesets. If you wish to include your own checks, you can put your checks in the rulesets Local_check_relay, Local_check_mail, and Local_check_rcpt. For example if you wanted to block senders with all numeric usernames (i.e. 2312343@bigisp.com), you would use Local_check_mail and the new regex map:

     LOCAL_CONFIG
     Kallnumbers regex -a@MATCH ^[0-9]+$
 
     LOCAL_RULESETS
     SLocal_check_mail
     # check address against various regex checks
     R$*                    $: $>Parse0 $>3 $1
     R$+ < @ bigisp.com. > $*     $: $(allnumbers $1 $)
     R@MATCH                    $#error $: 553 Header Error

These rules are called with the original arguments of the corresponding check_* ruleset. If the local ruleset returns $#OK, no further checking is done by the features described above and the mail is accepted. If the local ruleset resolves to a mailer (such as $#error or $#discard), the appropriate action is taken. Otherwise, the results of the local rewriting are ignored.

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Many thanks in advance