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.
@@ -14,6 +14,7 @@ protocol imap {
# Space separated list of plugins to load (default is global mail_plugins).
#mail_plugins = $mail_plugins
+mail_plugins = quota imap_quota
# IMAP logout format string:
# %i - total number of bytes read from client
ifaist0s