A postfix error that occurs when a user's mailbox is full.
Regularly caused by cronjob output, particularly when there are systematic errors in the cron job that lead to long and frequent email reports.
mailbox_size_limit and virtual_mailbox_limit may be relevant.
You can use
postconf | egrep '(mailbox_size_limit|virtual_mailbox_limit)'
...to see what size they are currently set at, which is probably 50MB.
Set this to a higher limit (in your .cf files), or even to 0 to remove limits.