Along with CentOS 4.2 came MySQL 4.1, great news ! BUT…
My old Courier-Imap version was only compatible with MySQL 3.x..
I had to upgrade to Courier-imap 4. My buddy kindly compiled the packages for me and published it on his repository : http://repo.securityteam.us/repository/
A few issue then came up with my imap configuration, I noticed a few changes had been made to Courier-Imap about MySQL connectivity..
As explained here http://blog.wains.be/?p=7, mysql configuration files of courier used to be under /usr/lib/courier-imap/etc..
The new version of courier requires the files to be located under /etc/authlib or courier will just complain about mysql.example.com not being reachable :
authdaemond: failed to connect to mysql server (server=mysql.example.com, userid=admin): Unknown MySQL serv)
You just need to move your configuration files into the new directory and you are done.. It took me 20 minutes to figure out the relocation of the files :)
More info : http://www.courier-mta.org/authlib/