Renan,
I am using RBL checking on SME 5.1.2, but I was able to do it on my test 5.6 box. After installing Charlie Brady's e-smith-mailfront rpm, edit /var/service/smtpdfront-qmail/rblsmtpd.conf (for 5.6)
Lookup on line how to edit rblsmtpd and you should be able to filter against any rbl sites you choose.
I did not have to deal with 'ucspi-tcp', but my test box is 5.6, not 5.5.
This box is not in use, but it passes the rbl blocking email tests from ordb and sbl spamhaus.
Once Mitel figures out PPTP and IPSEC masq and in/out traffic, I will upgrade to 5.6. Hopefully soon!!!! I want to upgrade!!!!
good luck
ryan