To monitor POP3, check the log in the /var/log/courier/pop3d folder.

root@hostname:directory# cd /var/log/courier/pop3d 
root@hostname:/var/log/courier/pop3d# cat current 

To monitor IMAP, check the log in the /var/log/courier/imapd folder.

root@hostname:directory# cd /var/log/courier/imapd
root@hostname:/var/log/courier/imapd# cat current 
2007-04-11_04:33:49.36893 INFO: LOGIN, user=siddharth, ip=[192.168.1.25], protocol=IMAP
2007-04-11_04:47:06.97295 tcpsvd: info: status 6/30
2007-04-11_04:47:06.97297 tcpsvd: info: pid 16445 from 192.168.1.128
2007-04-11_04:47:06.97298 tcpsvd: info: start 16445 0:192.168.1.5 ::192.168.1.128:52380

The file shows that a user has logged in using an IMAP connection. Observe the line tcpsvd: info: status 6/30. This indicates that at this point, there are 6 active IMAP connections out of a maximum of 30.