Plesk has a built-in limit of 4 concurrent connections to IMAP from a single IP, so if you have more than 4 users – or more than 4 accounts which connect back to your Plesk server from any 1 IP – your users will get a Send/Receive error.

After spending a long time trying to diagnose the connection error, we found it was simply a case of changing the “MAXPERIP”  limits within “/etc/courier-imap/imapd“. After changing the IP limit to something more suitable for your requirements, you will need to restart the IMAP service :

/etc/init.d/courier-imap stop
/etc/init.d/courier-imap start