I do NOT get any quota warning messages in zimbra (Release 7.2.0_GA_2669.RHEL6_64_20120410002025 RHEL6_64 FOSS edition.)
I created a test user and set his quota limit to 5 MB and sent a few mails.
under user's advance tab, i set it to ( Account quota (MB): 5 ) and save.
mailq show below output
(host mail.example.com [x.x.x.x] said: 452 4.2.2 Over quota (in reply to end of DATA command))
What's wrong with it.
I found below URls
Quota Warnings - Zimbra :: Wiki
The above is for ZCS 4.5