We have a server CentOS server setup which hosts our websites and email server (using POSTFIX). We manage the system using Virtualmin and Webmin respectively.
In general for every site that we setup we provide the customer with an email account on our system. One customer in particular has decided it a good idea to send out literally thousands of email in bulk. When this happens the mail server will become unresponsive and mail for other customers will go down whilst the queue is processed.
Is there a way we can limit the rate of email is sent for one particular user of the system rather than everyone? Is there also a setting we should change so that nothing a user can do can cause the mail server to start failing for other users?