Black lists, also known as RBLs (real-time blackhole lists) or DNSBLs
(DNS black lists) lists of hosts (Definition)
maintained by volunteers. By consulting these black lists, our servers
can reject spam attempts from listed hosts.
Although black lists are generally automatically enabled by default, you
can switch their use on or off. (See Adjusting Spam Filters.)
Almost all mail rejected due to the use of black lists will usually be
However, black lists do not selectively block spam. Rather, once a host
is listed on a black list that we use, all mail from that host, whether
spam or non-spam, will be rejected.
For more detailed general information about black lists, please see this