@Satoriya:
Hey, that's exactly my problem. I'm on Plesk 8.4 (migrated from, 8.3, 8.2, 8.1, 8,0 and 7.54) and Plesk made the quota-changes in /var/qmail/mailnames/DOMAIN/MAILNAME/Maildir/maildirsize.
My old mailboxes had the /var/qmail/mailnames/DOMAIN/MAILNAME/@mbox.quota - file and the...