Just create the (empty) file "locallowercase" in the courier 'etc'
directory (/etc/courier for rpms, /usr/lib/courier/etc for source
I tried this, and it seems to work for accounts in /etc/passwd, but NOT for
virtual accounts using postgresql authentication.
Just a guess: configure postgresql so that it performs case insensitive
matches on text fields (I don't know how to do this with postresql, but it
should be possibile since most DBMS allow this).