The Mail Abuse Prevention System maintains three blackhole lists which can be used to block email which is likely to be unwanted.
This tool checks whether a host is in any of those lists
The RBL is a manually maintained list containing those hosts which are major sources of unwanted email or which provide services to those who send unsolicited bulk email.
The DUL is a list of many dynamically assigned dialup addresses. These systems shouldn't normally be a direct source of legitimate email.
The RSS is a list of systems which will relay email for anyone and which have at some point been abused to relay spam