I remember way back, i'd modified the dspam training plugin to compare mailbox names case-insensitively. that patch wasn't accepted. i didn't understand why since it was obviously correct and had no negative impact, at least i didn't think so.
Now I've hit another case where case-insensitive mailbox names would be useful: +addressing.
If I have a mailbox 'Ext' and I send mail to frank+Ext@, the LHS is lowercased to 'frank+ext' and the +extension delivery doesn't find the 'Ext' mailbox.
If the +extension is going to be lowercased, then the mailbox names should be lowercased when looking for them. Or, since 'ext' and 'Ext' can co-exist, the +extension shouldn't be lowercased when looking for the mailbox.
I prefer the former (lowercase everything), since it's unlikely a user would create both 'mailbox' and 'Mailbox', however I can envision plenty of times where users would try to use '+ext' yet create the '+Ext" mailbox and never be able to figure out why it doesn't work. I know the latter solution (case sensitive extensions and case sensitive mailboxes) is more correct but really it will be harder to debug user problems.
-frank