wtf? cyrus is a mail server - it handles pop and imap requests (depending on setup and config), is not a proxy and stores mail for various users who are setup... now you tell me what that makes it!!!
+ i am not aware of any of the popular mail servers being autonomous - they all require at least an mta! (hmmm - i think postfix is one)
as for postfix and quotas check out this:
http://www.postfix.org/addon.html#quota
this was by the way returned by the search engine on postfix.org when searching for quota!!
have a read...
what mail server are you runing?