Hello Ihab please have a look at the quota_grace feature. It allows you to deliver that last mail which brings the user over quota. You will need at least Dovecot 2.2.x for that feature.
http://wiki2.dovecot.org/Quota/Configuration#Quota_grace We solved the problem by configuring a second lmtp service on a different port which is not affected by quota limits and is only used to deliver quota notifications. This has been previously discussed on this list. Regards Daniel
