Reducing Syslogd Performance Issues
Syslogd is the main logging program for many Linux systems.  Unfortunately it can cause performance issues by using synchronous writes by default.  If you open syslog.conf you will see that the mail logging goes to /var/log/mail.log.  In order to prevent synchronous writes a “-” is placed in front of the log.

mail.*                          -/var/log/mail.log

This is a default on most Linux systems so you do not have to do anything but it does provide an explanation for why the log file is different in the syslog.conf.