dragffy.com

Initially I had set a mail account with a quota of 200000, or roughly 200MB. The user complained of a full mailbox, despite using Plesk to up the limit to 2GB, and then unlimited the account continued to bounce emails saying:
"Recipient's mailbox is full"

I found people complaining of a bug in version 7.5 of Plesk, but it seems this is also present in 8.3. The problem, I discovered after nosing around the config files of the server, was that any changes I made in Plesk weren’t saved. Ultimately I found the limit in the config file remained at 200MB, despite numerous changes. The quota config file is located at:
/var/qmail/mailnames/example.com/username/@mbox.quota

Manually edit that file so it has the quota you want. I like to use vim:
vim /var/qmail/mailnames/example.com/username/\@mbox.quota
(run this as root)

After editing, save the changes, and you won’t need to restart any services or the server. Changes should take effect.

Note, that this was performed on a CentOS 5 installation, so paths and instructions may vary slightly on other systems.