Spam Filtering Lookups
Second that. As far as I know google doesn't use spamhaus.org but Yahoo and Hotmail can't protect their users and have to rely on third party help.
I was sending messages to a few clients and those with Yahoo and Hotmail didn't receive them. It turns out that my mail server was blocked by spamhaus.org.
It's not just spamhaus. There are other IP based and domain based blocklists. They will block you but they will not provide an automated tool to search their databases and alert you when you are blocked. Unless you create one for yourself or manually check their databases you have to wait for customers to report the problem.
I worked with support to clear this up and they gave me this address as the best tool out there to manually check against multiple databases if your IP is blocked: