The way I understand it is, this Quota service was built specifically for postfix. (I only have postfix, have not used any other MTA) The "quota-status" executable is in you libexec directory. ( I compiled my dovecot instance, hence is is not in "regular" directory)
Thanks. I also responded to Aki, but to close the loop: evidently quota-status is newer than the version of dovecot on my Ubuntu 12.04 machine. So I'll need to upgrade.
The "quota_status_*" are responses to postifx. AFAIK, these are the only 3 possible options.
Please see ( https://sys4.de/en/blog/2013/04/08/postfix-dovecot-mailbox-quota/ ) for a detailed info on this
When Postfix MTA is "inline" with the sender, and if you have correctly configured the quota service, it will check if the recipient's mailbox can accept mails. If the recipient is able to accept mail dovecot responds with "DUNNO" (as configured in "quota_status_success" ) to let postifx continue with its sender checks. if the recipient's mail box is unable to accept mails, the dovecot responds with "552 5.2.2 Mailbox is full" (as configured in "quota_status_overquota"), this will prevent postfix from accepting mail and will respond with 552 status . All this is documented in that blog.
Thanks. I saw the link to Hildebrandt's blog on the wiki. (BTW, his Postfix book is still great!) And I understood the example. But it didn't cover the answers to my questions. Aki covered most of them.
Thanks again, Michael