Thanks to everyone who assisted with this annoying problem. Not sure if i got it 100% right but my setup is much improved now.
As I was already using similar compile options, the fix for me was either the increase of MAXPERIP setting in /usr/local/etc/courier-imap/imapd (thx Matt) or/as well as more careful modifications of the configuration files as specified in http://www.jerfu.com/toaster/FullToaster_1.0.6.html (thx Ross). Instead of starting Courier with... /usr/lib/courier-imap/libexec/authlib/authdaemond start I now start with... /etc/rc.d/init.d/courier-imap start Thanks again.
