Using spamhaus on Plesk

I have a web server running Plesk, and I using "sbl-xbl.spamhaus.org" in the MAPS spam protection to attempt to block out spam mails. I do see a significant reduction in the amount of spam when I have it turned off versus when it was disabled.

Now, every other month, I will have a couple of clients complaining that they can't send out emails, as they were blocked by the spam protection. These clients are usually on dynamic IP, so a likely cause is that they have gotten a blacklisted IP. Usually, once they try to turn off and on their modem, and obtain a new IP address, the problem will go away. The problem is usually not critical, but it poses some slight frustration for some clients.

My question is, is this normal? How frequently do people experience such blockage when using SMTP servers using MAPS? Is once every two to three months normal? I have a couple of other email accounts with other service providers also using spam protection (or so they say), but I have not experienced this at all even when on dynamic IP. I only encounter this on my own server. Am I missing out something? Is there other settings in Plesk or other areas that I should be taking note of?

Thanks for any advice.