Increase Courier-IMAP Connection Limit VPS Server

by Mike Lugar
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It is very easy to hit the Courier IMAP connection limit on a VPS or dedicated server with Parallels Plesk. Plesk by default limits the number of connections to 4 per IP which is already one connection less than most email clients. Desktop email clients usually default to 5 simultaneous server connections in order to speed up mailbox caching etc.

This can cause unwanted error messages such as “too many simultaneous server connections” when attempting to send/receive email. This is especially the case when utilizing multiple computers behind one IP address through a single router/firewall. For example, having 5 computers running in an office with email clients utilizing the 5 possible default server connections each, is already 25 possible connections on one IP address, this is how problems occur.

To Increase the number of Courier-IMAP connections from a single IP on a Plesk CentOS based server, follow these simple steps:

Note:  Alternatively, you can use Perl find & replace, as illustrated in this Parallels KB article.

1) Log into your server via SSH

2) Open the IMAP configuration file with your favorite editor as follows:


3) Modify the MAXDAEMONS & MAXPERIP variables as follows:


4) Write the changes to the file and exit your editor

5) Restart the Courier-IMAP service


That’s it, now your total connections allowed per IP is 40, ten times the default of 4.

Mike is the managing member at MSLtek and is an avid web consultant and web developer. He enjoys solving complex problems and building awesome stuff.

Posted on March 28, 2014

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