By default, UNIX-based servers running Plesk and the Courier-IMAP e-mail server drastically limit the number of inbound connections to prevent users from opening up too many concurrent sessions. Unfortunately, this artificially-low 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.
Plesk comes configured with a limit of 4 connections per IP address and a limit of 40 connections total. Modern IMAP clients such as Mozilla Thunderbird use mailbox caching to open up multiple connections to increase performance. In the case of Thunderbird, it opens up 5 connections by default which is already 1 connection more than Courier-IMAP’s default restriction. Add another few family or corporate computers behind a firewall and those additional users won’t be able to connect at all since a single Thunderbird client is already utilizing all 4 connections.
To increase this restriction, modify the
/etc/courier-imap/imapd configuration file and change
MAXPERIP to a more sane number. In the case of my configuration, I changed
MAXDAEMONS from 40 to 80 and
MAXPERIP from 4 to 40. This allows all the machines behind my home firewall to connect to multiple accounts on the e-mail server with mailbox caching enabled.
But even those numbers may be too low for a corporate colocated server that services an 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.
Obviously, the connection limits are to prevent the Courier-IMAP server from using too many 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.