Spamhaus tracks the Internet's Spammers, Spam Gangs and Spam Services, provides dependable realtime anti-spam protection for Internet networks, and works with Law Enforcement to identify and pursue spammers worldwide.They have three sub project, SBL (SpamHaus Block List), XBL (Exploit Block List), and PBL (Policy Block List). Each project will have different policy, so i suggest you check all three of them.
phpBB engine used SpamHaus to check every post posted to the engine. If the IP are listed on SpamHaus, it will be blocked and the request to post will be denied. OK, enough with theory, what's the solution?
They blacklisted IPs which are identified related to spams, virus distribution and many more. Why on earth a valid IP can be blacklisted by SpamHaus? There are several reason. Most obvious reason is that one or more computer on that IP has been compromised or infected by a virus/worms which spreads spam messages, so the spam message comes from that IP address.
When your IPs are blocked, you will get a warning when you tried to post. It's something like this: Your IP aa.bb.cc.dd has been blocked because it is blacklisted. For details please see http://www.spamhaus.org/query/bl?ip=aa.bb.cc.dd.
You can start by visiting the page (change aa.bb.cc.dd with your real IP address) and you will see which project blacklisted your IP address. You can start working on fixing the problem when you know what's wrong with your IP. When you have fixed them, you can start asking the SpamHaus to remove your IP from their list and you will be able to post again on the forum. It may take hours to days to remove your IP, so be patient