Many site administrators (and the persons they work for) are interested in the volume of mail passing to, from, and through the local site. There are a number of ways to quantify mail traffic.
If you use mail.debug and get any medium to high mail volume, the syslog output can get quite large. Output files from syslogd generally need to be rotated or purged on a routine basis from crond(8).
There are a number of commonly available utilities that can summarize the output of mail logging from syslogd. One of the more well known utilities is syslog-stat.pl, a perl script that is distributed with the sendmail+IDA sources.