Using a quota rule like "quota_rule = *:backend=1073741824S" (I'm testing it statically but the value is on LDAP) I receive an an error:
Fatal: IMAP(...): Quota root QUOTA: Invalid rule: *:backend=1073741824S Accordingly with the wiki page http://wiki.dovecot.org/Quota/New "Backend-specific configuration currently is used only with Maildir++ quota backend. It means you can have the quota in Maildir++ format (e.g. "10000000S")." so I think it should work. What's wrong? Cheers, Laurento