Error: Courier-IMAP - imapd: Maximum connection limit reached for

You cannot connect to Courier-IMAP and see the following errors in your mail log:

 Aug 08 11:25:45 server1 imapd: Maximum connection limit reached for ::ffff:1.2.3.4

Solution

Open your Courier-IMAP configuration file; on Debian it's /etc/courier/imapd:

vi /etc/courier/imapd

Adjust the MAXDAEMONS and MAXPERIP settings (you might have to play around with these values)...

[...]
##NAME: MAXDAEMONS:0
#
#  Maximum number of IMAP servers started
#

MAXDAEMONS=40

##NAME: MAXPERIP:0
#
#  Maximum number of connections to accept from the same IP address

MAXPERIP=20
[...]

... and restart Courier-IMAP - on Debian this can be done with:

/etc/init.d/courier-imap restart