Hi, I have the same Problem, here the mail.err:
Dec 16 13:19:04 webserver check-quota[1133]: cannot get sender domain
Dec 16 13:19:04 webserver check-quota[1133]: Unable to intialize check-quota mail handler
Dec 16 13:19:04 webserver plesk sendmail[1132]: Error during 'check-quota' handler
Dec...