Since Parallels Plesk Panel version 8.4 it is possible to limit maximum number of recipients for email message. To prevent your users from sending mass e-mail, do the following:
1. Create a file named maxrcpt in the directory $QMAIL_ROOT_D/control/ where $QMAIL_ROOT_D is the location defined in the file /etc/psa/psa.conf file.
2. Type the number of allowed recipients in this file and save it.
Note that this number also affects sending of messages to mailing list or mail group subscribers. That is, if you set the value to 100, then only 100 subscribers will receive the message sent to the mailing list or mail group.
When you no longer need to restrict the number of recipients, delete the maxrcpt file.
For QMail you can try the following:
1. Recipients Limits
Then set your own number eg 50
Then: /etc/init.d/qmail restart
2. Stopping QMail Double Bounces
and add ‘#’ in that file. NB: only #