#
# ---- For iRedMail-0.7.4 and later versions ----
#
mysql> USE vmail;
mysql> DELETE FROM used_quota WHERE username='[email protected]';

#
# ---- For iRedMail-0.7.3 and earlier versions ----
#
mysql> USE vmail;
mysql> UPDATE mailbox SET bytes=-1,messages=-1 WHERE username='[email protected]';