A user was getting this error when trying to send out 600 messages (that’s a separate issue)

Message Denied – Your IP address has sent the maximum number of messages per day. To reduce the possibility of spam, your outgoing message has been blocked.

on Plesk 9 + Ubuntu, edit Config.php – /var/www/atmail/libs/Atmail/Config.php

change the “filter_max_msgs” setting to suit your needs…