I'm currently using a virtual MTA system, including amavisd-new, spamassassin, clamav, postfix and using quotas on a per mailbox basis. When I upgraded to 2.4.5, I started seeing quota limits exceeding on accounts that have quota limits of '0'. Zero quota use to mean no-limit.
Nov 25 07:55:41 comp postfix/virtual: DF03FA38D: to=<firstname.lastname@example.org>, orig_to=<email@example.com>, relay=virtual, delay=0.18, delays=0.07/0.02/0/0.1, dsn=5.2.3, status=bounced (maildir delivery failed: Sorry, the user's maildir has overdrawn his diskspace quota, please try again later.)
Now this is very puzzling. Checking out Google, I discovered that the VDA patch has changed it's handling. Now, '0' quota will fall back to virtual_mailbox_limit setting in /etc/postfix/main.cf. This was originally set at 30MB (314572800 bytes), and that was the problem. Cranking this up to 300MB should be enough to handle this, but I've seen configuration examples where users are setting it to 1GB (1073741824 bytes)
Here is my post on the Gentoo forum
Looks like postfix-2.4.6-r1 fixes this issue. Here is the bug report.