cPanel
All cPanel logs are located in the folder:
/usr/local/cpanel/logs
The access_log contains all traffic to WHM, cPanel, and Webmail:
/usr/local/cpanel/logs/access_log
The error_log contains all errors when accessing a cPanel related site:
/usr/local/cpanel/logs/error_log
Apache
On cPanel servers each site has its own traffic log, but Apache's main access log can be viewed here:
/usr/local/apache/logs/access_log
To view each cPanel accounts individual access-logs:
/usr/local/apache/domlogs
Apache errors are logged to the error_log file:
/usr/local/apache/logs/error_log
Exim
The exim main log contains all transaction information:
/var/log/exim_mainlog
The reject log contains all connections that were rejected:
/var/log/exim_rejectlog
FTP
Regardless of the FTP server chosen in cPanel, it does not have it's own log and instead logs to:
/var/log/messages
SSH
On most servers the SSH service will log to the secure and system messages log files:
/var/log/secure
/var/log/messages