I can confirm this as well .... I had one person that hit there quota.
They could no longer access there e-mails till I raised there quota so they could delete some old mail.
Also the web mail interface while trying to log in comes up looking strange wile they are locked out.
Never bothered to put in a bug report or even check as it only happened once in 3 years.
Bad boy on my part I guess

Regards,
Tib