RFC Ignorant (RFCI) is a blacklist. It lists sites that don't have working postmaster or abuse addresses, among a few other reasons. The name "RFC Ignorant" comes from the belief that not having these addresses working on your system means that you're out of step with Internet RFCs (Requests for Comments), the guiding documents regarding Internet interoperability.
It's on my mind lately, as I found a site I assist listed on it. It's easy enough (in this case) to fix the perceived issue and get the site delisted (removed). I'm more interested in resolving issues than starting a fight with a blacklist operator. So, I'll just deal. Still, it's surprising to me that the list is even still around, as I recall that the disagreements over its value (or lack thereof) started shortly after the list was created, way back in 2000 or 2001.
Not a lot of people use this blacklist, because it really has very little to do with spam blocking. Lots of sites send no mail, or have some reason that they wouldn't have a postmaster or abuse address. But, even if a site's reason for not having such an address is a bad reason, it still doesn't mean they're a spammer.
So, if you use the RFCI blacklist to block mail, you will potentially block mail from some sites that violate RFC guidelines, as interpreted by the blacklist operators. But, that doesn't mean it'll block any spam. Lots and lots of spam-spewing sites comply perfectly with the RFCs, and wouldn't be eligible for listing.
Here's one example of the kind of disagreement the RFCI blacklist engenders.
Here's another, slightly more calm take on why a site wasn't able to continue using the RFCI blacklist.