On a busy server, the default number of connections allowed for IMAP users might be too low. If you have IMAP customers complaining about connections being refused, you might need to increase the number of simultaneous connections.
Edit the following file:
/etc/courier-imap/imapd
Restart imapd:
/etc/init.d/courier-imapd restart
If you have a server that is not responding, there might be an attack on one of the domains. You can get a good idea if a normally low volume website is suddenly getting lots of traffic by running a few checks on the log files.
Count the number of posts to each unique file:
# grep POST /usr/local/apache2/logs/USER/DOMAIN-accesslog | awk ‘{print $7}’ | sort | uniq -c | sort -n
Count the number of times each IP posted to the domain:
# grep POST /usr/local/apache2/logs/USER/DOMAIN-accesslog | awk ‘{print $1}’ | sort | uniq -c | sort -n
Count the number of unique IP addresses that posted to the domain:
# grep POST /usr/local/apache2/logs/USER/DOMAIN-access_log | awk ‘{print $1}’ | sort | uniq | wc -l