After moved user's mail inbox from old server to the new one, soon we experience imap quota reported wrong to the current inbox size of user email. We try to figure out whats really happen and finaly got to know that Maildir/maildirsize on every user email folder updated in a weird way. When an email come in it write the info of email size twice (normally just one) but when an email deleted from inbox then maildirsize just update normally, so only one record of size email marked as deleted, this lead to wrong calculated email quota which make everyones confused!
Google show many links that prove some bodys out there experience this problem too, but nothing really get solve the problem. Here we think this is imap quota related problem, but obviously this is vpopmail related problem. I found this link and the problem get solve when i downgrade vpopmail from currently installed version 5.4.25 to the older vpopmail 5.4.23.
I have posted an email to courier-imap list tryng to get help from them. There more info about what really happen to my problem.
Update 9 Feb 09
Today the problem still reported from another client, after some investigating finally i found that if i pop without leaving message on server or i delete messege via pop3 command then file Maildir/maildirsize didn't updated. This behavior came from vpopmail used new way namely maildir++ and the old qmail doesn't adapted it (yet). So there is a patch which mean need to recompile qmail, i just doit this noon and the problem solved.
Don't forged to remove all of the Maildir/maildirsize file, don't worry it would recreated when an email come in, so the quota info not screwed again. I did like this:
find /home/vpopmail/domains/ -name maildirsize -type f | xargs /bin/rm -f