Sendmail log files are complicated and hard to analyze. They have the IP address of the connection in one place, and the results of the connection on another line later on. Correlating the two, so you can make a list of IP addresses that are misbehaving in a certain way, is difficult.
This filter is intended to make the correlation easier. It takes a standard sendmail log file as input, and produces as output the same log entries but with the IP address appended to each line, where possible.
See the manual entry for more details.