This extension will display the current status of your
IMAP quota in thunderbird's statusbar and will warn you
when you reach a configurable limit.

It only works with IMAP quotas and servers advertizing
the quota status via IMAP (the IMAP GETQUOTA command).
Some mail servers have disk quotas and will send email
when your mailbox reach its quota. That's not a real
IMAP quota system and the imap server might not advertize
it.

WWW: https://addons.mozilla.org/thunderbird/881/