Applies to :
By default, in a Linux email server installed with Plesk and the Courier-IMAP, it allows a limited number of inbound connections to prevent users from opening up too many concurrent sessions. The defaults is 4 connections per IP address and a limit of 40 connections total.
This restriction can impact legitimate users who have multiple computers connecting to the Courier-IMAP server from behind a firewall or a single computer that runs an IMAP client that takes advantage of mailbox caching.
You will see this error when you try to accessing your email :
To rectify the problem, you need to login to the server or VPS with root privileges via SSH.
To increase this restriction, you can modify /etc/courier-imap/imapd configuration file and change MAXDAEMONS and MAXPERIP to a reasonable number. For example, MAXDAEMONS increased from 40 to 80 and MAXPERIP increased from 4 to 40. This allows more machines behind the users firewall to connect to multiple accounts on the e-mail server with mailbox caching enabled.
However, those numbers can be too low for a corporate colocated server that serve the entire company. Tweak those numbers based on your employee base; if 50 employees are connecting to the e-mail server from behind the same firewall then MAXPERIP could need to go as high as 250 (50 employees times 5 cached mailbox connections). Add e-mail clients of people working from home and MAXDAEMONS could go as high as 300 or 400.
The connection limits can be used to prevent the Courier-IMAP server from using too much memory and CPU resources on the machine. Tweak the numbers based on the memory footprint of each daemon process and how much memory you have.