I wanted my users to see the quota on my IMAP server on frontend login. I couldn't find such a plugin, so I wrote it myself. It consists of two parts: wogri_imapquota, which is a service, that hooks into the typo3 authentication mechanism, and gets launched during login. This is because I don't store my frontend user passwords in the local typo3 database, but in a ldap tree (I authenticate using ldap_auth). The second part visualizes the results of the authentication service.
The bad thing is: If you can't read php code, you won't be able to "configure" my extension. It's not userfriendly at all, and you need to read my helpful comments in the sourcecode. Won't take you longer than 5 minutes, the service part is very short.
If you have problems using it (or if you succeed using it), let me know!
My IMAP Server that supports quotas: Cyrus IMAPD, though might work even with exchange, if it sticks to the standards. It supports SSL encryption upon imap login.