Some changes are needed to enable the showing of quota showing in Roundcube and other IMAP clients. See http://forum.vestacp.com/viewtopic.php?f=16&t=9197 for details.
@@ -78,6 +78,7 @@ protocol pop3 {
# Space separated list of plugins to load (default is global mail_plugins).
#mail_plugins = $mail_plugins
+mail_plugins = quota
# Workarounds for various client bugs:
# outlook-no-nuls:
ifaist0s