We maintain a small blacklist of mail addresses that we will reject mail from because they have caused problems in the past.
Generally the way this blacklisting works is that we will accept connections from the blacklisted sites and reject their messages during the attempted mail transfer. This allows us to send informational messages back to the remote mail system, detailing reasons why their mail was rejected. In extreme (though, thankfully, relatively rare) circumstances, we encounter a site that is so abusive that we can't handle them like this. They may, for example, open dozens of connections to us simultaneously, in an attempt to get as much junk through to us as possible before they are noticed. This has the side effect of denying service for other mail connections. In cases like this we may have to simply deny connections to our relays altogether. This is the one case where a remote site will not be able to get mail through even to postmaster or abuse, which is permitted even for blacklisted sites, so they can make contact with us.