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...
I now start with...