This plugin will check and display users' mail quota status. Current and
maximum quota usage is displayed in easy-to-read graphical format. Optional
warnings may also be displayed to users who are nearing their quota allocation
when they log in (where the "Message Of The Day" would normally be displayed).

This plugin is compatible with three types of mail quota systems:
UNIX (filesystem), IMAP-based, and cPanel quotas.

WWW: http://www.squirrelmail.org/plugin_view.php?id=237