Log lines duplicated in syslog and mail.log

All log lines in /var/log/mail.log are also being written to /var/log/syslog. This seems like a waste of space. Is this intentional, and can anyone confirm whether this is happening to you as well?

Easiest way to see is by tailing both log files at the same time:

tail -f /var/log/syslog /var/log/mail.log