maildirquotad is used in conjunction with exim. Exim has no builtin facility for checking the quotas of a mailbox at RCPT TO time. And when you have many MX servers and only one mail hub that stores mail data, it became a real problem, for example, if an MX server has received mail but the mailhub defers it because recipient's mailbox is full, despite the fact that the MX server already has said to remote server that mail had been successfully delivered.