After installing an ISPConfig deployment, everything seemed to work properly, but every now-and-then I got this weird error that there was something wrong with the mail server configuration. The Apple Mail.app showed a exclamation mark with the following message:
The server returned the error: The attempt to read data from the server server.domain.ext failed.
Some research showed that the Apple mail clients tend to open several connections for IMAP, and the default setting of the Courier IMAPd server is to allow (only) 4 connections from the same IP address.
Modifying the Courier config file (/usr/lib/courier-imap/etc/imapd) and allowing e.g. 20 connections from 1 IP address solved this problem.
<ORIGINAL CONFIG>
##NAME: MAXPERIP:0
#
# Maximum number of connections to accept from the same IP address
MAXPERIP=4
<MODIFIED CONFIG>
##NAME: MAXPERIP:0
#
# Maximum number of connections to accept from the same IP address
MAXPERIP=20
If your company / household holds several imap mail clients you may need to increase the counter even more (65536 is the maximum amount of connections for ANY IP address).
If you have SSL enabled on the Courier IMAPd server you also need to add the MAXPERIP variable to the imap-ssl config file (/usr/lib/courier-imap/etc/imapd-ssl).
Finally, you need to restart the Courier IMAPd services (/etc/init.d/courier-imap restart)