There are (at least) two other ways to accomplish what you wish, @rogeriobrito:
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sender dependant relays in postfix will let you use different SMTP servers for different domains and in extreme situations, different email addresses, should such granular configuration ever be required.
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mail rate limits of Virtualmin let you define an upper limit for messages that can be sent out per hour / per day for every domain / virtual server and this protects your server and IP address from excessive abuse even if hackers gain access to a mailbox and use it to send spam.
2 is easier to implement than 1. I suggest you go with 2 first and if it does not satisfactorily protect you from abuse by hackers who spam, then think about doing 1.