courier mta can be compiled with trashquota option where deleted files and files in trash folder are counted into quota (they are not by default).
Please add this option for courier (courier-imap 4.6.0 has this option already).
Created attachment 257317 [details]
adds trashquota option to courier ebuild
this patch adds trashquota option to courier-0.65.2 ebuild.
compiles with and without this option, seems correct.