Using spamhaus on Plesk

I have a web server running Plesk, and I using "" 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.
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There really isn't a 'normal' to compare to... When a client is using a shared IP address system, they have NO CONTROL over what other people who have previously obtained that IP address have done.  All it takes is one spammer on the system (or even someone with a virus!) to get an IP address banned, and they will then start to randomly get blocked.  They really need to take note of the IP address, and contact their ISP - the ISP who needs to know about this, and take appropriate action with Spamhaus and/or the other people who have used that IP address.

Using this sort of system, unfortunately, is a brute force method - you're in, or you're out.  Other spam programs take a different approach, where they rate an email on the liklihood of being spam, and take action based on rules.  The most popular of these sorts of systems are Bayesian-based systems such as SpamAssassin, SpamBayes, etc.  Systems that use Bayesian-based spam blocking can use MAPS and Spamhaus as one of many things to take into consideration when deciding whether or not an email is spam.

I'd expect that these other providers aren't using the brute force method, and are likely to be using a Bayesian method.

(Unfortunately, I'm not a user of Plesk, so I really can't advise you on that behalf.)
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